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Thanks guys :o)
Haven't had a response from Telefonica but I did speak to someone at Equiniti (previously Lloyds TSB Registrars), very helpful.
I certainly don't remember it (I was in the process of finding a flat to rent temporarily / getting my stuff in storage / buying a house end of 2005 / beginning of 2006, was an absolute 'mare, hence long story !) but the guy read me a letter I'd written in Dec 2005 saying I'd take part of the sum in cash, and the remainder as a loan certificate. I'd have to go thru' bank statements from that time to prove it (and I've no reason to doubt him, the letter he read sounded genuine), and he also said he wasn't aware of a 6 year deadline - gave me another number to ring to confirm that - but he is writing to me re getting a cheque issued as redemption of the loan certificate (minus £50 admin charge - I can live with that !) ! Result !! :)