There is finally a happy ending to this. On Friday morning I went to the LA and made it clear that I wanted a deadline by the following week, after which I wanted to pull out at zero cost, otherwise I would have held them responsible for the costs incurred in case the letting had fallen through, and was going to seek legal advice in order to safeguard my interests. Then I rang in the afternoon, and they said "Ironically, the moment you walked out of the office, the landlord's wife called saying he'd been ill and working very long shifts, and apologised for not signing earlier" (BTW, typically