Wow, that's a lot of info. Thanks, we don't pay you nearly enough, you know.
Because the fault's intermittent, it's hard to 'prove' any of the sypmtoms, but I did notice that an XP laptop, running IE6, consistently opened
www.diy.co.uk several times, during which time my XP desktop, running ie7, consistently failed. Both are working fine now, for that website at least.
I'll wait and see how demon respond - hopefully they'll just tell me they're dealing with the problem. Then again, I recently called them to suggest that neither of their DNS servers was working. On that occasion they seemed surprised by my suggestion and insisted on getting me to answer all sorts of pointless questions about my router status, before eventually admitting that they already knew their DNS wasn't working.