Nothing's straightforward is it? Just when everything is sorted, my prospective supervisor gets a new job elsewhere. Any idea where I stand? Do they have to find me someone else, as the offer has already been made?

Thanks :)

hi barnaby, it's really never all sorted. but good luck for sorting this next obstacle!
did you apply at that place specifically because you want to work with that supervisor? then it might be an idea to go with him/her, that is, keep the supervisor, change the university. if he/she helps, that should be possible even now, i think.
if you want to stay where you are, then look at the staff profiles, and contact those people who seem interesting. maybe your former intended supervisor can help you with that. i do believe the department WILL provide you with another supervisor. you might get someone temporary for a while in the beginning, until the replacement for your intended supervisor is found.

Contact the institute, only they can tell you.