I know what you're saying @JSHarris, but this
are a bit at odds with each other I doubt many lay people would attempt to replace a boiler main board.
But, it could be that the boiler could be repaired (parts and labour) for under £3-400
I thought that it was up to you to ensure that you put only the right things in the recycling? If something is not recyclable then where should it be put? No the recycling bin surely? We are constantly told that putting the wrong things in the recycling spoils the entire load.
If I was having a bonfire anyway I would likely burn it but as I have a bonfire about once a year mine ends up in the wheelie. I wouldn’t start a bonfire just to burn rubbish that I could stick in the wheelie.
FWIW, I replaced the main board in our Vaillant 831 EcoTec some time ago. Board was around £120, IIRC and replacement took at the most 30 minutes, and that was for an amateur who had never changed one before and had to read through the second level diagnostic instructions as I went to make sure I programmed in the correct DSN number, to programme the board to work with the EcoTec 831 (they use the same main board for several different models, so it's supplied un-programmed).
At £900 new, i hope someone has contacted the cowboy to cancel, or at least ask him to justify both the need for replacement and the £900 fitting cost. And why he charged a call-out fee (though I guess he could say the callout was free, it was £80 for the diagnostic check - do you know how long he was there for?)
If it does turn out the boiler can be fixed with a replacement part, i'd also be contacting local trading standards.