My employer has realised they have overpaid me for several months, due to calculating my pay over 52 weeks rather than 38 weeks. Actually there was some overtime and holiday which wasn't paid which reduced the overpayment significantly. However, they've now come back to me again and said that they have overpaid me because they've been posting my 20 minute break. They've gone back to September and calculated the total I've been overpaid in breaks (ironically, I've never had a break, I just do the admin relatedd to my job and often come back after 15 mins as I assumed it was paid break), removing that also from my next wages. Are they allowed to do this?