@Motheroftwofeline: once bitten twice shy!
I think you need to balance protecting your interests, vs pissing off the seller, but understand if they pull out they might be liable for basically nothing (£100 for EA photos, poss nothing for conveyencing)
Is it a buyers or sellers mkt in your area? How hard would it be to find another house?
So I would: be looking at other houses with other agents (selectively) - you might find one where its mid transaction and honestly that you like better. I cannot imagine agents ALL talk to each other
Meanwhile, get a commitment from your seller of when they move/by when they would decide to move to a rental if they cannot find BEFORE you pay for survey, & also appoint a conveyencer who won't charge too much before you move
I would make booking your survey conditional on seeing the sales memo on the house that they are moving into (or firm arrangments to move elsewhere). This could be an issue if eg they plan to move in with family, but I really thought it was fishy that sister's vendors taking forever to find a place: if you really want to move you will take some action (& start getting rid of things)
If they won't agree to that, ask them before booking survey if they will cover the cost if they pull out - that would help you gauge it
See how things go, also make an excuse to go back and see the property in a month or 2 - if you see that they have continued decluttered/have boxed stuff up its a VERY good sign. People I am buying from had already started boxing stuff up when put house on mkt (they are relocating) which made me confident they were actually planning to go
How long are your buyers prepared to wait? are they FTB/ how important is the SD break for them. If the SD thing gets extended to end June, and I was offering on a house know I really would be wanting it to complete sharpish (& I would personally look elsewhere if chain blocked in about 2 weeks)
How much do you want to keep your own sale? would you rent if you had to? Or buy a compromise house to get SD break instead of this one? These are things to think about as your vendors will be looking out for own interests, not yours
I was really sad for my sis (& the expense of survey was a lot for her to lose as basically a FTB/not high income), BUT I also think she did not protect her own interests in this situation, which you have a chance to do