Anyway, to answer the questions:

It is a solid state amplifier.


Most components are soldered directly to the board or have flying
leads.

Tweetldee:
The components in the power supply are definately thermistors, at
least, according to the circuit diagram I have (could be -ve slope
though as you say)

I disconnected the preamp stage inside and the noise persisted. i then
tapped components along the circuit path to see if any affected the
noise (very scientific I know but I have no equipment beyond a
soldering iron!). A couple of electrolytics seemed to make the noise
worse/better when tapped them so I figure I'll replace these first.
They are feed through capacitors on the small driving (before the
power) transistors.

The problem is farnel/RS don't stock these value of cap. Can I replace
these with 2 or more caps in series/parallel to make up the value i
want?

Cheers

Dean