In 1998, the government sold a tranch of loans to Finance for Higher Education Ltd, a holding company set up purely for the sake of investing in public student loan debt. It seems that your loan was included.
Tricity Lombard Finance were appointed to "oversee the administration of the portfolio" for FFHE. They're not debt collectors and they don't own your debt. (But if you get any problems with your loan account, I don't see why you couldn't approach them.)
Just to confuse things, FFHE was established (and may well be owned) by NatWest. LTF does not appear to be a company in its own right (it may be a trading name), but is owned by The Royal Bank of Scotland Group.
SLC should have told you about the sale in 1998, although I don't know if they're required to. If asked, SLC may say (incorrectly) that the loan was sold to LTF, as this is who they deal with.
Hope this helps. If you want any more info, let me know.
SC
HOWELLS ANNOUNCES RESULT OF STUDENT LOAN DEBT SALE