Northern Rock redemption fees

Essex32

Has anyone had any bad dealings with this company in relation to ending their mortgage with them and switching to a new lender.
Basically I got a "illustration only" redemption figure in December and now Im switching to another lender, asked for a up to date redemption figure and understand the early penalty figure, admin costs and giving up deeds, but the figure I have received for ending the mortgage has increased from December to March by over GBP 1200.. I have made 3 mortgage payments (jan/feb/mar) so if anything I expected the figure to come down.
When I contact NR asking why the figure has increased by so much they said its the monthly interest calculated. I do not understand these interest fees. I asked them the payments I make surely they include interest if so why are you now trying to charge me these 3 interest payments, I was put on hold and whether they cut me off but as I work after 30 mins no one came back. Before I write them a letter please can you advise if these interest payments are correct ? I was paying GBP 650 per month they calculate the interest which they have added to the final redemption figure of GBP 450 a month. The mortgage I have with them is a repayment.
Also can anyone advise when asking for a redemption figure what charges should be calculated apart from the penalt fee.

Thanks