How stressful and horrible, really sorry to hear this OP.
Unless you live in a stately home, it does not cost anywhere near £50k to replace garden walls and treat a loft for mould. Or to unblock a drain (not sure what that costs but it should be cheaper than a reline, and to reline our damaged under house drains we’ve been quoted £1800).
They have produced a very suspicious round number. For £50k you could, on a 4 bed house, take the loft off, throw it away, and build a brand new lovely 2 bedroom loft conversion (we had quotes recently, a bit more expensive than anticipated! We’re in south england).
That they haven’t shown you the survey yet is also suspicious. It definitely will have a valuation number in it. That number might be disappointing and surveyor valuations can vary greatly. But you should be told what the official valuation is.
I don’t quite get how the garden can be sinking and “need relandscaping”. If you have a subsidence issue that is damaging your house, then that could indeed lower value, but that doesn’t seem to be what they’re saying. I’m sure if their surveyor mentioned subsidence they would have told you!
I think the starting point is to write to them with something like the following...
“Dear X... We’re sorry to hear that your surveyor has raised some concerns about the loft and garden. We’re shocked by his comments, as I’m sure are you. Would it be possible for us to see the written report, so we can reflect on the detail?
At the moment our thinking is that, when the house was valued at ££, that was in its current state with the loft and garden in their current condition. If we had relandscaped the garden and updated the loft, the original asking price would have been higher to reflect that. [For example, for £50,000 we could convert the loft into a new floor with 2-3 rooms which would significantly raise the value of the house.]
We are still keen to go ahead with this sale, but we simply aren’t in a position to do so at the figure you mention: it wouldn’t fund our intended purchase or justify the investment we’ve made here. For that kind of price it would make more financial sense for us to hold on to this property for now.
As I say we would very much like to sell to you, but if not we will need to take advice and possibly put the property back on the market. Given the timelines we’ve discussed, I would be very grateful if you could let us know as soon as possible if you in a position to proceed with the exchange on [date] at the agreed price?