@Joanna it is 4 weeks, I worked 3 years July this year
I'm not sure what advice to give, it's in the contract that you'll have to pay back OMP if you don't return for the minimum term I'm afraid, although it may be able to be in any role within the NHS rather than your original role? But double check that
Speak to your union rep or HRBP. Yes, the contract will say you have to pay it back, but often caveats exist like they cannot make you destitute in recouping the money. You can speak with payroll and arrange a repayment plan. They aren't entitled to just take it out of your money.
You have to legally work the 3 months to be able to keep your maternity pay. How long is your notice period?