We've been experiencing issues for 3 - 4 weeks on sainsburys.co.uk on both Macs at home - my mini and my wife's MacBook running 10.5 - Safari up-to-date, reset, cookies cleared. Mostly on the groceries section; I was able to order some electrical items earlier this week with no problems.
Having finally got through to the technical side earlier this evening, I was told that the problems (which appear as redirection to a 404 page or just looping to 'werror.html') have been reported to Apple and a fix is awaited. I did point out that perhaps they should make customers aware without having to phone!, but this fell on stony ground.
The current advice is to use Chrome or Firefox on Macs.