It is difficult to say if it's a big job or just something simple - it will probably cost more than you expected whichever it turns out to be.
Some boiler engineers are very good and some just make guesses at your expense - they may tell you that more than one part was at fault to justify replacing several items. Some repair companies - the larger outfits - get rich on "up-selling" - encouraging you to have supplementary work done at great expense.
Before you engage an engineer you will do well to ask about his diagnostic skills - ask him the direct question, "Do you just change things willy-nilly until it works?"
Ask him (her), "If you change an expensive item like a £300 gas valve for instance - and it's not that causing the problem - who pays for your diagnostic error?"
The first thing any engineer will do is to check to see if there's a "call for heat" i.e. Is there a signal from the timer and the room thermostat asking the boiler to fire up? Then he will check the fan etc in a logical process.
He will not say, "it's most likely the timer" until he's sure.
Good luck...
Peccavi, March 2013
