On Fri, 22 Jan 2016 12:11:31 +0000, Chris wrote:

It can be so hard to tell sometimes. Try scoping the output from the
transformer (taking suitable precautions of course!) and look for any
irregularities that might indicate internal arcing.

Well I've just scoped the B/E junction of the chopper transistor and it's
just ALL noise; no discernable control pulses at all, so clearly this
thing has issues beyond what this medium can assist with, I'm sorry to
say.
Anyway, must do some shopping right now or there'll be hell to pay.