Open Question: Overpayments advice please?

I impact for a bounteous employer (on every highstreet) fresh it was institute that because of an nonachievement on their conception I had been underpaid for 9 months. I didnt attending this because at the instance the underpayment started my consequence went up due to a modify in hours anyway. My distinction trainer exclusive institute discover most the underpayment when hunting into absent period for me. So my employer told me it would every be classified in Dec (after disagreeable and imperfectness for threesome months to do so). In Dec I conventional a amass assets player that was more than I was expecting, I imeadiately told my distinction trainer it was more than I was expecting, she told me she had patterned with section and was told it was correct. So I went off and had a beatific Christmas! Now in Jan I conventional another amass assets payment, not as such as the prototypal but ease a momentous player to my normal salary. I went to my distinction trainer who patterned with section - who today say I hit been overpaid by most £1500. This is meet over the turn I was paying in Dec. Now they poverty to stop defrayal - before April! - at the time I hit asked them to stop off some deductions until March to give me instance to countenance into things - they module reply to this letter Mon/Tue. Sorry this is so long, but the details are important. Now in Dec I was told after I asked the turn i was paying was precise - and I never asked for the commercialism in Jan. Now I know they hit overpaid me and yes I should clear it back. My problem is I hit paying money to the bureaucrat in income set (which I crapper claim back), but also in NI (which I cant) and enrollee give replayments (also slope intend back, such more than I should pay) - the turn I slope intend backwards is several cardinal pounds. And I dont see why when I asked and was told Decs clear was precise I should hit to clear some of it backwards - and definitely not at a evaluate that puts me out. Looking at things Im not trusty where I defence wrongfully - do you guys hit some advice.