Not until you part with the £270 - then you may have been conned...
A PCB might cost as much as £200 for some boilers but usually closer to £120 - it might take as much as 20 minutes to replace it.
The engineer making the diagnosis, driving to the parts depot, buying the part, driving back and fitting it and then testing the boiler for correct operation and any other safety checks he deems necessary or is required by regulation to conduct (or thinks he is or manages to convince you that he is) - could easily take a couple of hours to do this. Allow say, £30 an hour for his time and £150 for the part then you're up to £210 - VAT might bring this up to £240 or so.
Some boiler engineers, just like the rest of us, sometimes tend to guess the most probable cause - they are not usually too clued up on electronics and the control board is an easy fall back for them and very often they're right.
A key question to your engineer might be, "who pays if it isn't the control board?" Some naughty engineers will just replace parts at your expense until they stumble on the true cause - they will claim that several parts had all broken down at the same time which is most improbable.
You do not mention the make or model of your boiler or the failure symptom - you are perfectly entitled to buy and replace the PCB yourself unless you're a Landlord.
Do not let others try to scare you by quoting safety regs...
The Gas Safety (Installation and Use) Regulations 1998
can be found here...
http://www.opsi.gov.uk/si/si1998/19982451.htm
Of interest is the following from PART A GENERAL
(6) Nothing in these Regulations shall apply in relation to
(c) work in relation to a control device on a gas appliance if -
(i) the device is intended primarily for use by a consumer of gas; and
(ii) the work does not involve breaking into a gasway.
Good luck...
Peccavi, August 2009
