I think this constitutes less favourable treatment for you as a part-time worker. The following extract applies to bank holidays, but I have put in bold the bit that suggests you are being discriminated against if they close for the hours you normally work, according to your contract, but then penalise you financially for not working then. I reckon a letter to HR pointing this out should do the job. Perhaps flowerbeanbag could check I am right and help with the phrasing?
www.desktoplawyer.co.uk/dt/browse/law/index.cfm?fuseaction=ViewContent&sid=75850&aid=35412
Public holidays and bank holidays
The rights of part-timers in relation to public holidays and bank holidays may not always be clear.
Under the regulations, part-timers should not be treated less favourably than comparable full-timers in their entitlement to public/bank holidays. Allowing full-timers the day off, but not part-timers, is clearly less favourable treatment and unlawful under the regulations unless there is objective justification.
To comply with the law, an employer must treat part-time workers as favourably as they treat full-time workers. In some circumstances, it may be enough simply to give workers a paid day off if their day of work happens to coincide with the public holiday, without giving time off in lieu to those who would not ordinarily work on that day. This may produce a fair result, for example, where a shift system means that full-time and part-time workers are equally likely to be scheduled to work on a public holiday. However, where workers work fixed days each week, such a practice could put part-timers at a disadvantage. Since most bank and public holidays fall on a Monday, those who do not work Mondays will be entitled to proportionately fewer days off. In many workplaces, these workers will predominantly be part-timers.
In such cases, it may be necessary to remove the disadvantage suffered by those staff who do not receive particular days off as a result of their particular working pattern, for example, by giving all workers a pro rata entitlement of days off in lieu according to the number of hours they work.
Whether either of these approaches meets the requirements of the regulations will depend on the particular circumstances. Whatever approach they choose to adopt, employers should bear in mind the principal that it is unlawful to treat part-timers less favourably than comparable full-timers unless there is objective justification for doing so.