Ask your insurers what they want exactly, and if they have any criterion regarding what surveyors they will accept:
"Full out of water"?
"hull and deck structure only"
Then with this information have a phone around the local surveyors in your area to see what they can offer.
should turn up a couple in your area.
I think that Bosun might be a little off in pricing at £10/ft for a proper surveyor though.
Would agree that bringing the boat to the surveyor will save you money - especially if the fee is to be kept low. A simple survey outside the surveyors office on a windy October Wednesday when he's quiet could be a good "fill in" job.
What boat is it -can't be many that are 20 years old - apart from seariders!
As an aside there may be web based insurers that don't need a survey for a 20 year old boat. - The requirement varies depending on market conditions - it used to be 10 years of age 20 years ago.