It would be very risky to pay a deposit in those circumstances. Even if the survey goes through, there could be a whole host of other reasons you wouldn't want to buy - local searches disclosing problems with access rights, easements, obligations to support. Not clear if the property is freehold or leasehold - if leasehold, your solicitor would need to fully review the terms first. Were they proposing a contract be actually signed, as otherwise my understanding is that the deposit would be returnable in any event (since any contract for a sale of land must be in writing, or is voidable at either party's instance). Your solicitor should be able to confirm if this is in point here. Bear in mind that to pay a deposit, there needs to be a contract and consideration, part of which is represented by the deposit.
The downside of not having it in writing is of course it doesn't really protect you, since contract would still be voidable at seller's instance.
P.S. - whilst I am a solicitor, I specialise in corporate tax rather than property law, so check all of the above with your own solicitor and do not rely on it!