If the credit agreement is a regulated agreement under the terms of the Consumer Credit Act 1974 then, from what you say, it appears that the credit agreement has been 'improperly executed'. An improperly executed credit agreement is unenforceable. And yes, you should also have received a copy of the agreement back in 1990.
What that means in practice is:
Not only are you entitled to the goods. You are also entitled to receive any other costs - e.g. past payments, interest, any default charges, etc etc etc.
Wilson v. First County Trust
Went all the way from the County Court all the way up to the House of Lords.
http://www.publications.parliament.uk/pa/ld200 203/ldjudgmt/jd030710/will-1.htm