It's a 1920s build that's had one end knocked down and a modern (2005-ish) extension built. We're likely to go with a building survey (which I think is the highest RICS registered surveyors offer) but have no idea about how to pick a good firm. Similarly with conveyancing, there are so many options!
Well a 20’s house will be either solid or cavity construction with timber suspended floors and a rafter and purlin roof. It’s the later extensions that can cause problems as they often cause bridging between old and new for water, block air bricks etc - there can be issues with high ground levels, dodgy sprayed foam insulation to the under side of the roof, cracking due to replacement double glazing - the list goes on. Oh and asbestos is prevalent but provided you know where it is, it’s general condition and don’t mess with it, it won’t hurt you - you can then organise it’s renoval safely. Often around pipes, old bacolite switch gear, garage roof coverings etc - don’t run a mile if some is found. It’s very common - just quiz the surveyor.
The big firms are e.surv, Countrywide, Shepherds etc. Or you can go for a smaller independent firm - best to google but make sure they are RICS accredited.
With solicitors people are often very quick to recommend online companies that are basically the cheap option. This is fine if you just want a very basic service and are happy to pray that nothing a bit abnormal comes up (such as rights of way or planning issues) - it’s at this point that people often wish they had put the hand in their pocket and paid for a proper solicitor.
Bare that in mind and if in doubt use a company that is local and long-standing. You must have someone you can talk to during the process - not some 18 year old legal clerk in a call centre in Manchester.
On the flip side, these companies can be good at saving some cash for you when you are buying something very simple.