This isn't just an AS issue it seems to be an occasional glitch - I had the same message on a manual bid 4 times - including after a reboot and then everything went fine (and I won)
There have been some occasions when pages are also unavailable and links from the watch list don't work etc. - also odd routing when trying to place items on watch...
My best guess is that there is some 'noise' corrupting some of the data on EbayUK... any other thoughts?