Transcom chasing but I've paid!!

Hi all, long time lurker first time poster!!

I had a County Court Judgment order me to pay Arrow Global LLC £3357.00 by instalments of £50.00 per month starting August 2008 until its paid off. Fair enough, I stuck my head in the sand and put on my big boy pants and faced the music.

I have paid £50 per month from a standing order to them and later Transcom when it changed to them.

Scroll forward to 7th March 2014 and I've paid, including this month, £3400, so actually £43 over the amount, but now I've had a letter from Transcom saying I still owe them £850.30 and discuss a settlement discount and discuss my financial circumstances.

I haven't contacted them yet in case it triggers some kind of acknowledgement or anything like that, so I just wondered if this is common practice, or what the heck they're trying to do!!

Any help muchly appreciated.