The SLC seem to be a bunch of crooks.
My wife has had 1 payment left of £160 for about 3 years. And has regularly phoned up/written to them asking to pay this final payment. They finally got in touch on the 27th of June to say they were going to take payment from her next salary, so she phoned them the next day to pay off the final amount which she did, and they said they would issue a "stop notice" on the account.
Fast forward 5 days (yes less than a weeks notice), they took a percentage from her salary which was significantly larger than the £160,