Baxi Genesis 80 not firing consistently?

My boiler occasionally fails to fire for both heating or hot water - it is inconsistent. I called in the engineer and he mentioned a circuit board and thermistor, and said that he would need to get a quote as he he did not keep the spares to hand.

I was contacted by phone with a quote of £383 plus vat for the parts, which included the following: a power module, flow switch regulator, two sensors and a control module. I've researched these items on the internet and the most I can cost them at is around £195. The engineers wanted payment up front for the parts!

I have asked for a quote for a boiler replacement and an itemised quote for the repairs (the latter has not yet been provided). I am a little concerned that potentially I could be hugely overcharged for this work. Can anyone advise?

Many thanks