Boiler on the blink

Hi all a bit of advice is needed, i have a saunier duval combi boiler (sd 623). It decided to break down on new years day! At first we noticed the pilot light had gone out, so we set about relighing it by holding the ignition switch in etc, this worked for about 10 mins and the pressure gauge went from 1bar to 4bar when it shut down again.
we have had a guy come and look at it who said the thermocouple had gone and the expansion vessel and that we should get a new boiler! eek.
Another guy came out and said he could fix said problems, so we thought it would be better to try and get a few more years out of it while we save!

He spent all last night trying to fix it, he drained the system and he pumped up the expansion vessel with a foot pump to 0.8bar and it seemed great, the pressure then stayed on 1bar. he still said i needed to order the thermocouple as it wasnt a standard one? so i did and it is due to arrive tommorow.

My question is before he started messing about i could get the ignition switch to remain in while just the hot water was on so we could shower etc and it would only pop out after the heating had been on about 10-20mins but now i cant get the switch to stay in at all! and if i hold it in with a knife (i know this is not what im supposed to do) the heating will fire the boiler up but if i then try and run the hot tap it all knocks off (even though im holding in the switch)
Does anyone think this is a new problem since the last plumbers been in there or is it still a thermocouple problem (which he is coming back to instal when it arrives).
Any help/advice would be appreciated.
Nadia.

Does anyone think this is a new problem since the last plumbers been in there or is it still a thermocouple problem (which he is coming back to instal when it arrives).
Any help/advice would be appreciated.

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give the lad a chance to finish the job

Hi and thanks for the speedy reply, of course i will let him finish the job but i wanted to be armed with some information in case he was also just trying to get us to buy a new boiler from him. I can just see him coming up with "new " problems as it never seems to stop at just one or two and i wanted to know if they will be genuine problems. Obviously i only want to go down the purchase a new boiler route if it is absolutly nesessary. (fingers crossed)

I am also trying to educate myself about my boiler but there is a lot of stuff to learn in a week and i still have to trust the prof opinions of another person who may/may not be taking the proverbial.