Hi. I would walk, save your money, such sellers are toxic trouble down the line - survey costs are a nice tropical winter holiday, send them a postcard. Luckily, this is not the only house on the market, there will be another. This is where the HIPS pack (remember those - Seller pays into a survey etc into one easy package, until it was eviscerated to a thermal walkabout, was actually doing something sensible weeding out the toxic time wasting flakey sellers - don't let them cost you anymore than your time to date.
Hello back on HPC after many years.. finally bought earlier this year (to live in) after a long process (4 months) that should have been quicker (probate sale) when I went to collect the keys the senior partner at the estate agent told me that an institutional buyer had tried to gazump at 15% over the agreed but very luckily the seller stuck to his agreement with me after some wobbly thought- so it does happen. Had offered on the property on the first day and was the third to view property (and had later approached asking on vendors request), made an offer on the spot as had been attending