OK, I thought I'd better post what appears to be the solution.

Getting increasingly frustrated at not being able to get my PC to talk to my 80 Titanium, I went through all the settings options one by one toggling them on and off in case there was somethning in there inhibiting the connection........and there is.

In settings/developer options I switched off USB Debugging and hey presto, the PC now not only recognises the tablet, but generates a very pretty little tablet icon on the 'My Computer' screen with Archos 80 Titanium written next to it.

I have no idea why USB Debugging should kill the connection, but it sure does.

PS:   OOooops, sorry, this should have been attached to my original thread 'Help for Nooby'.  Mod please remove if you wish and I'll put it where it should be.