Baxi Combi 105e - Central Heating Sensor Fault

My Baxi combi boiler has recently started cutting out. The flashing light is for the central heating sensor. The boiler wont fire up if this sensor is flashing. Sometimes if I turn it off and reset it then it will work when it's switched back on, sometimes not.

I have no technical knowlege of boilers or the like. I'll get an engineer out to look at it but he'll have to come when i'm at work which is a shame as i'd like the chance to chat to someone who knows about these things.

I had someone come to look at the boiler earlier in the year as the pressure kept dropping, he was a bit of a cowboy and just turned the tap to allow the pressure to rise without looking further into the problem - he was in and out in 5 mins and happily pocketed his call out fee. Naturally, i had to call out another engineer to fix the fault as the pressure continued to drop.

Any reccomendations for Edinburgh based engineers would be gratefully received.

You need a thermistor part No 5108265 (£10ish) a 13mm spanner a philips screwdriver and 10 minutes!

Where pray tell do i pop this thermistor into?? Do you unscrew the front panel? I can be pretty handy with a screwdriver but am very new to the thermistor game.
thanks for all help

Refer to your Installation and Servicing Instruction book, available on line from partsarena