Many thanks for your helpful suggestions; MAC discussions are dominated by helpful and polite people thankfully. I will remember that important tip re resolved versus unresolved threads.
If I may, for the benefit of any one stumbling across this with a similar problem, here is how eventually I resolved it. I spent abut 40 minutes with Apple Care on this and we tried a lot of things - emptying caches, reinstalling Safari, repairing permissions, verifying disk. Nothing seemed to work. Ultimately, it was faster and easier to erase the account (ensuring any important files were safe) and just recreate it. Of course, this is not a great option for everyone, in my case, as there was little that needed saving on the second account, it was faster. I am still curious as to the origin of the problem.
Cheers