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Well I have now uninstalled Adblock, but it is no better.
I am sure it is in some way related to the adverts, as gov and BBC websites seem to run fine.
When it is hanging, a little box in the bottom left corner says 'processing request'. That box normally flashes up the various tasks it is performing, so it looks like it is arriving at a task and not being able to do it.
I'll just have to go back to using Firefox, though that annoys me the number of times it stops for updates.