> Alternatively you could simply pay the money for a high-voltage
> op-amp, e.g. the TI OPA445

That's a new one on me, but I see it's available from RS. Previously
I'm sure their best last year was other OPA @ 44V that I found for
a PSU, so I didn't check in the latest catalogue for anything better

> A switch mode design seems like massive overkill in this case.

In one way perhaps yes, although there will still need to be a 48V
supply made from the 12V. A simple opamp design should do for
this, and maybe leave a PIC-based SMPS as a personal exercise
for another day
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