In the absense of a friendl person, I would send them a letter like this:
"Thank you for your letter of X date.
I would be very grateful if you could explain how the figure of £2,000 has been calculated.
I was paid 90% of my salary for six weeks, 50% for 6 weeks and Statutory Maternity Pay for the six months remainder of my Maternity leave. I understand that the amount I need to repay is anything over and above the statutory minimum. By my calculations this is as follows:
Annual salary: £21,000
Weekly salary: £403.85
Weekly salary @ 50%: £201.93 x six weeks = £1,211.58
Minus Statutory Maternity Pay (£117.18 per week X 6 weeks = £703.08) = £508.50
I therefore calculate that I owe you £508.50. I would be grateful if you could explain how the figure of £2,000 was calculated, so that I can arrange repayment of the correct amount.
Many thanks for your help. Yours etc."