remember on labour only, you have given a price assuming all material will be on site and fit for purpose, so also include 2 things with your quote, 1 a daywork rate of £x per hour for hold ups caused by wrong/damaged/short material, and 2 a call out charge of £x as you supplied labour the only prob that will be a freebie will be caused by your faulty labour which you can control, so when you visit after a call to say no heating you get paid for the call and subsequent hassle when you visit and find the prob is the new boiler, you didnt supply = you dont warrenty, if i supply and fit, i cover all visits and if the boiler i supplied is faulty i will organise the warrenty visit etc