(Some of you may laugh and say this is obvious, but I hope it may help someone, somewhere along the line)
In the last week or so, I have been doing quite a bit with my web site.
By "coincidence", in the last week, I have been unable to get past the log-on page at my bank and unable to check my online invoice at 1899.com.
Speeds have also been a bit grim at times, despite Cricklewood exchange being sorted a few days ago - it didn't make a jot of difference - and I've had the odd page that just refused to load properly and cause a freeze-up.
Then it dawned on me. Because of doing a load of web page creating, I had left the validator permanently on, with the errors and warnings totals showing in the status bar, instead of disabling it in the browser and just enabling it when doing pages offline or using "View page source" to check a page.
Disabling validation in the browser solved *all* the problems.