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OK, update time. I have no recollection of being served with a Windows disk, but as soon as I turned on it told me to make a recovery disk (major clue here). Not one of my wide range of DVDs was accepted, and in the end I gave up, and forgot about it. Bad move but inevitable. I suspect in the absence of a personal copy of Windows (despite paying for it) that was the replacement. Too late now obviously. There was also no code with it either.
I contacted HP (who now own Compaq) who informed me they won't authorise any models they didn't test for Windows upgrades for them so it is blocked from 8.1, possibly permanently. The model is a CQ2000. I will now have to call Curry's tomorrow (on a Saturday...) for their stock non-service dept. solution, as this must happen all the time and there must be a plan B when there is no Windows disk (which I paid for) or recovery disk (which not everyone is likely to have made however many times it reminds you). But I'm sure there's a 50-50 chance had I possessed either of these I may not be waiting for the letters to appear on my ancient hand made PC I keep as a spare.