If you want a big estimate as bargaining leverage, go with a free survey who will almost certainly recommend remedial work. The firm our buyers chose (who also ended up on watchdog or rogue traders more recently) also recommended raising the timber floor and treating for woodworm as they found one woodworm hole in the roof and a couple under the stairs. Downstairs they said the floor was concrete but its suspended timber as can clearly be seen by the floorboards under the stairs For about 5 metres of solid external wall treatment and woodworm treated of loft and under the floor of two small rooms they quoted £2k.
On the other hand, pay an independent person for survey for an unbiased assessment, and perhaps discuss with vendors the benefit of this over a firm looking to make a quick buck, and they may be happy to go halves. As sellers we even considered paying for our own independent survey but in the end due to time constraints, offered to cover half the quote, or they could walk away.