I don't understand about how your holiday is factored in??? You are entitled to paid holidays and paid bank holidays just like fulltime workers. It sounds as if you don't have any paid holidays, and that is illegal.
If they didn't pay you any maternity pay then you certainly don''t have to pay it back - actually I'll check the law on that.
In the meantime, this:
Part-time workers
The majority of part-time workers are women and therefore any less favourable treatment of part-timers will have a disproportionate impact on women.
Examples of discrimination in pay against part-time workers
Being paid at a lower hourly rate, or less than the pro-rata salary, of full-time workers
Having less access to other pay benefits (such as occupational pension, bonuses, shift pay, sick pay) compared with full-time workers
Receiving less than pro-rata entitlement to non-pay benefits (such as holiday entitlement) received by full-time workers.
Requirement to justify different treatment of part-timers
It is now beyond doubt that a pay practice which results in full-timers being paid more than female part-timers is prima facie discriminatory unless it can be objectively justified on factors unrelated to any discrimination on the grounds of sex. For example:
In Bilka-Kaufhaus GmbH, part-timers had been excluded from joining an occupational pension scheme on the basis that employing full-time workers involved lower ancillary costs and facilitated the use of staff throughout opening hours (the employer in this case was a department store). The ECJ held that this did not amount to justification in the circumstances of this case.
In Rinner-Kuhn v FWW Spezial-Gebaudereinigung GmbH [1989] IRLR 493, the ECJ said that an employer cannot justify excluding part-timers from benefits on the grounds that they are less connected and committed to the organisation than full-timers.