I would charge £60 diagnosis but because a replacement cylinder is a big enough job I would say if you go with me I will knock the price of the diagnosis off the install. But obviously add a little bit on like I do with all my quotes to cover my time pricing.
My way of looking at it is if they phone up and say they want a quote and say it is a cylinder change I will happily go out and do a price for them. If they phone up and say i have a problem can you come and have a look i will say no problem it is £60 to diagnose the problem i will give you a price to repair it.
I had one a couple of months ago I went out to do a breakdown on a Saturday morning (I was running late on the friday so had to cancel) when I got the it was low pressure so I topped up and checked expansion vessel etc and they were on about wanting a new boiler, as I had only been there for 10 minutes and I felt guilty I couldn't get to them the night before as she has kidney problems (she is proper yellow) I said as you want a new boiler I will give you your quote and wont charge you for today. They were saying they were expecting the quote to be around 2k i came in at around £17-1800 and every time i go past the house now still see the same boiler in place. So basically i was played for a fool but when the boiler goes down again i wont put myself out to get there.