The timeline so far:
20th April: Builders say they'll finish at the end of May. I figure that this realistically means early June, so I arrange to rent my room here until the end of June.
28th May: Builders say that it's now going to take a bit longer, but they should be finished by mid-June. I tell them that I want to move in on 25th June, and they say that they will definitely be finished by then.
9th June: Builders say that being finished by 25th June is going to be tricky, and they could really do with another week (i.e. up until 2nd July).
11th June: I tell builders that I may be able to rearrange my plans, but that if I reschedule everything for 1 week later I don't want to repeat the process again after that. They wouldn't like to commit to 2nd July, but say that they will be finished by 9th July.
So, my current plan is to move in on Monday 12th July. This is all rather frustrating (as well as being time consuming and expensive), but unfortunately my options are rather limited. Still, it's a learning process, and the most important lesson I've learned is "next time, DIY".