I very much doubt this is a pipex-specific issue as in reality I suspect you don't have a pipex connection, you have a BT connection TO pipex with some kit at your end.
Could it be that the kit is running quite warm and in the hot weather it fails? Or that your BT line is sensitive to this? You say it "gives up the ghost" but what does that mean? Can't connect whatsoever for hours? Or it drops the connection?