Relief of the poor in England in the early 18th century was based on the Poor Relief Act of 1601 (the Elizabethan Poor Law). While its implementation has been the subject of much study, less attention has been paid to its assessment, apart from the frequent complaints about the burden it placed on communities. Dorothy Marshall, the former Preston Park School pupil, would seem to provide the best guide,  and the Preston Poor Tax Book of 1732 gives some insight into how the tax was assessed in one town. The following information is taken from Dorothy Marshall’s book.
She writes of the suspicions of corruption that gathered around the collection and disbursement of the poor tax:
Throughout the period, but particularly in the latter half of the [18th] century, contemporaries were united in complaining that the parish officers were neither efficient nor honest. The truth of this statement it is difficult to prove, except in particular instances, but the law offered a considerable number of loopholes, of which men wishing to indulge in corrupt practices might avail themselves, while the very fact that the overseers were unwilling, unpaid, annual officials, must have made for bad administration. The average overseer was either a farmer in rural parishes or a shopkeeper in urban ones; he was engaged in earning his own living, and was generally unwilling to waste more time and thought over his troublesome duties than was absolutely necessary. It was to his interest to keep the machine running until his year was over, but not to start new experiments, which he would never have the opportunity to carry through, and which the next overseer would probably discard. Moreover, he was usually quite unqualified for his task, and in some instances could not even write, but was reduced to making his mark. One cannot write down all overseers and churchwardens as embezzlers and defrauders; the most to be said is that their circumstances did afford opportunities for fraud, of which, in many cases, they availed themselves. But more definite than this it is impossible to be. 
However, Preston’s overseer of the poor seemed to have served for longer, to judge by the same name recurring at different dates in the Preston Poor Tax Book.
Taxpayers appeared to have resented the indulgences to which the parish officers treated themselves as recompense for their duties:
Another cause of expense was the unnecessary feasting indulged in by the parish officers at the parish charge, whenever they had cause to transact any parish business. In the large urban or city parishes the abuse was more pronounced, as the feasts held there were of a more luxurious type … Since the parochial officers received no actual pay for their work, they intended at least to compensate themselves by a little jollity. 
Dorothy Marshall (on page 69) calls on Daniel Defoe to lend contemporary support to this view, ‘But there are too many Parish Feasts … the Spit is too often saddled, and the Bottle goes too merrily round, for the Ease of the Parishioners.’ 
Another of the contemporary concerns was the lack of transparency in the way in which the tax was assessed:
Up to the passing of this Act [an attempt at reform in 1744] the rates were inaccessible to any but the officers of the parish, unless they were produced before Quarter Sessions by order of the Court. This power gave officers unlimited facilities for making unequal rates, and for favouring some persons at the expense of others, since no one could know with certainty what his neighbour paid.
Another Act passed the same year, dealing with the same subject, ordered that all the rates made for the relief of the Poor should be entered into a book specially kept for the purpose, and that fourteen days after all appeals from such rates were determined they should be entered up in the said book. The book in question was to be kept in a public place, where all persons so assessed “may freely resort,” it was to be handed on to the succeeding overseers, and was to be produced at Quarter Sessions when any appeal concerning rates was heard and determined. 
The Poor Law did not provide for the appointment of paid officials, but some parishes chose to do so. The Preston Poor Tax Book suggests that the parish was one of them:
Besides endeavours to make the control of the vestry over the parish officers a real thing, another method of securing an economical and effective administration of the poor law was to appoint an assistant overseer with a small salary, who was to help the annually-elected overseers, so introducing some continuity of policy and knowledge into the parish. The parishes which adopted this method were always in a minority, and the practice itself was extra-legal, though it was a device which found favour in the eyes of the poor law reformers towards the end of the eighteenth century.
Manchester had a paid overseer from an early date; for when the question of a workhouse came before Parliament in 1730, a certain Mr. Bowker, who was “Overseer of the Parish and hath been so for Ten Years past, for which he receives a Salary of Thirty Pounds per Annum,” was called upon to give evidence.
Liverpool ‘ In 1743 the vestry appointed Thomas Wharton to be overseer, along with those duly elected, at a salary of £20 per year, and he appears to have done most of the administrative work in connection with the poor.’ 
The assessment and collection of the poor tax could give rise to a sense of injustice, real or imagined, on the part of those called on to pay. The 17th-century Preston diarist Lawrence Rawstorne could be found complaining about the burden placed on him by his responsibility for the poor tax on his estate, notably for Hutton where he owned the whole township, and voicing his suspicion that he was being treated unfairly. Dorothy Marshall writes:
The amount payable by each parishioner towards the support of the Poor was fixed by the churchwardens and overseers of the Poor, and this function placed considerable power in their hands. The original intention of the law appears to have been that each parishioner should pay in proportion to his ability to do so. When the population of a parish was small and stable, and when everyone knew everyone else’s business, this was not so difficult to arrange with some show of fairness. In many cases, however, rent became, at an early date, a rough calculation on which it was possible to base a tax that in theory depended on ability to pay. In other words, a man’s rent came to be regarded as an indication of his general financial position; and on this, as a result, was levied an equal pound rate for the relief of the Poor. 
 Dorothy Marshall, The English Poor in the Eighteenth Century: A Study in Social and Administrative History from 1662 to 1782, Studies in Social History (London: Routledge, 2007).  Marshall, 58.  Marshall, 64–65.  Daniel Defoe, Parochial Tyranny: Or, the House-Keeper’s Complaint against the Insupportable Exactions, and Partial Assessments of Select Vestries, &c. … by Andrew Moreton, Esq. [Pseudonym of D. Defoe.] (J. Roberts, 1727), 7, https://books.google.co.uk/books?id=sXtQnQEACAAJ.  Marshall, The English Poor in the Eighteenth Century, 68–70.  Marshall, 72–73.  Marshall, 79–80.