There has been an AM/PM preference trial during the summer alongside various part time working options which are going to be rolled out company wide next year.
Generally earlies are short(ish) and the lates are usually longer.
But no bidding system per se. You request what you want and you might get it then you're on it for the foreseeable. Although if you choose a part time option they can't guarantee a full time option will be available down the line.