Vir Campestris expressed precisely :
It's gone wrong, and the guy looking at it reckons it's had it. That's a
surprise to me... is it likely, or is he another cowboy?

Assuming it isn't repairable, what's the current view on boiler
manufacturers?

All boiler faults are repairable, bound to be parts available, but what
has to be considered is the cost versus likely life once repaired. What
has this guy said has failed on the boiler?

The more recent the boiler, the shorter their working life seems to be
and the more complex they become. First boiler, an old Glow Worm lasted
almost 30 years and repaired up a few times cheaply.

We were persuaded to replace it with a more efficient condensing Ideal.
That was more complex, more complex to repair and became uneconomic to
continue repairing it due to cost of parts versus a new boiler. Present
one is a Vaillant, installed a year ago, has much better diagnostic
abilities built into it and a neat display.