Unfortunately I'm down on the south coast.
Prices vary widely depending on how experienced and busy a surveyor is.
Have a look on Marine Surveyors & Marine Designers
for a surveyor near you.
For the keenest price I'd look at affiliate grade members - they will be perfectly competent but not so senior that they demand high fees.
Insurers vary on their policy as to when they want a survey. 25 years seems reasonable and they won't want a survey more often than every 5 years.
They are simply trying to ensure they aren't insuring basket cases.