Most parts are quite straight forward to change on these appliances for a qualified gas engineer, where you need to apply caution is diagnosing the stat as faulty without looking deeper into the internal electrical control box as these now 20+year old appliances can be prone to ‘printed circuit board degradation and dry solder joints’ can you hear the gas valve ‘chatter’ as it trys to fire up? This is a sign of pcb degradation, a qualified engineer will be able to expose the electronic controls within the appliance and gently rock the connection molex plugs in the sockets to see if this makes and breaks the electrical connections. Tests are to be done using a multimeter set on ohms for continuity and no power to the boiler.. don’t attempt this yourself which by the looks of your post you’re planning on doing.. ring a local gas safe registered engineer. Be safe, and don’t play ‘spare part surgery’ as indeed replacing the stat may very well leave you with the same fault after fitting. I charge £50p/h to fix boilers (including fetching parts from a local supplier) so expect a £75-£125 labour bill plus parts costs. Better still, consider changing the appliance as efficiency on this one is between 70-75%.