I had a second look at the one letter I have access to, and it says that the vendors specifically would be reponsible. I'd say you'd have very little luck persuading them to deal with it once they'd moved on!
Tellingly, the estate agent is resolutely refusing to reply to any messages about this. Anything else she'll answer, but this query gets radio silence. I suspect she's hoping to drag it out till the deeds arrive from the bank in 6 or 8 weeks, at which stage she thinks we'll be too desperate to move to back out.
Back to daft for a plan B...