The way I see it, is a survey would pretty much be a waste of time & money for you ol' timers who have been around boats & engines since you've been knee high to a grass hopper, you know what to look for and where to look for it.
However, for us folks who are new to both the world of boats & engines, a surveyor is a good investment.
For instance, when we were looking for a boat, I knew enough to open cupboard doors and sniff to identify if there was any damp smell, but I wouldn't have had a clue to ask if there was an inspection hatch in the main cabin to see if there was any signs of damp or water in the bilge. I wouldn't have had a clue how to check to see what condition the insulation was in or how thick it was, nor would I have had a clue what to look for when looking at the hull unless there were large gaping holes. I also had no clue about the proper amount of ventilation to look for and the list of things I didn't know just goes on and on.
So with that said, we were very clear with the gentleman selling the boat that our offer price would solely be based on the outcome of the survey - after all, we were paying for the survey, he was taking it out of the water to be blackened anyway so it wasn't costing him anything extra. He asked if we would mind if he was present during the survey so that he could rectify anything that was found on the day, even though he was fairly confident there wouldn't be much. Most things he was ale to sort out whilst Trevor was there, there were a couple of jobs he was unable to complete on the day as he either didn't have the tools or parts, but sent our surveyor pics of the completed works within a couple of days.
Since he completed everything highlighted on the survey we were happy to pay the asking price which as it happened was a few thousand less that what the surveyor said the boat would be worth in his opinion. As it stands, boats of the same size, condition and age as ours are still being advertized for a few thousand more than what we paid for ours 3 years ago. And if the boat is anywhere close to London, then we are talking over 10 - 12K more For some folk, 10 - 12K isn't a lot of money, for us it is.
But that's just our experience, I appreciate others are different.