If you have taken legal advice then I am not in position to challenge.
What I can do is explain my rationale and if you feel it stacks up take it back to your
The Part Time Workers (Prevention of Less Favourable treatment) Regulations 2000www.legislation.gov.uk/uksi/2000/1551/regulation/5/made
“Less favourable treatment of part-time workers
5.—(1) A part-time worker has the right not to be treated by his employer less favourably than the employer treats a comparable full-time worker—
(a)as regards the terms of his contract; or
(b)by being subjected to any other detriment by any act, or deliberate failure to act, of his employer.
(2) The right conferred by paragraph (1) applies only if—
(a)the treatment is on the ground that the worker is a part-time worker, and
(b)the treatment is not justified on objective grounds.
(3) In determining whether a part-time worker has been treated less favourably than a comparable full-time worker the pro rata principle shall be applied unless it is inappropriate.
(4) A part-time worker paid at a lower rate for overtime worked by him in a period than a comparable full-time worker is or would be paid for overtime worked by him in the same period shall not, for that reason, be regarded as treated less favourably than the comparable full-time worker where, or to the extent that, the total number of hours worked by the part-time worker in the period, including overtime, does not exceed the number of hours the comparable full-time worker is required to work in the period, disregarding absences from work and overtime.”
I have highlighted the relevant paragraph.
The government advice www.gov.uk/holiday-entitlement-rights is that
“Irregular hours - People working irregular hours (like shift workers or term-time workers) are entitled to paid time off for every hour they work.”
If full time is 37.5 hours and get 28 days statutory leave x 7.5 hours = 210 hours @£15 per hour. Note: 37.5 x 52.18 weeks = 1956.75 annual hours
A part timer working 25 hours ( 5 hours a day) is entitled to 210 /37.5 x 25 = 140 hours @£15 per hour. Note: 25 x 52.18 weeks = 1304.5 annual hours
If the part timer works 10 hours extra for 16 weeks = 160 hours their annual hours + 1304.5 = 1464.5. So leave entitlement becomes 210/1958.75 x 1464.5 = 157.17 @£15 per hour. 17.7 hours paid leave x £15 = £265.5 less paid leave.
The op may have a part time contract of fixed hours (25) but they are working irregular additional hours (not overtime) which over a year = 1464.5 /52.18 = 28 hours a week or 5 hours 36 minutes a day. Each time they take a days leave they are paid 5 x £15 rather than 5.6 x £15 so effectively their hourly rate is £13.39 which is less favourable than a full time employees.
Note: this does not apply to overtime as full time employees don’t get extra annual leave.
The basic premise is does a part timer get exactly the same £ and benefits for each hour worked as a full timer does. For the odd extra hour it’s not an issue but 10 hours a week for 4 months is.