Another update to my story for those who haven't been bored to tears with my saga...
After measuring the correct PSU voltages under load, I thought perhaps the problem may lie in the front unit (possibly the capacitor for the voltage regulator?), so I decided to dismantle it. Firtly I have to agree with other posts I've seen in this forum - the radio unit apears to be a superbly constructed piece of kit, which is in stark constrast to the PSU construction in the rear.
If you take your time it's fairly easy to disassemble the unit, and I managed to have the main CB separated in about 40 minutes. I had a good nose around but couldn't see anything obviously wrong (no bulging caps or burnt resistors). I was interested to see that the CM2596 voltage regulator circuit, and the 470uF capacitor matches the spec in the datasheet which I found here: http://pdf1.alldatasheet.co.kr/datashee ... M2596.html
I decided to re-connect the power connector only, with the intention of measuring the input and output voltages on the CM2596. However, as soon as I did this the controller booted up and the familiar ROKU scrolled into view on the display! After receiving this surprise I didn't bother to measure the voltages, but gradually reassembled the unit, connecting each piece (spkrs, switch unit, etc) in turn and then re-connecting the power. Each time the unit started successfully and it wasn't long before I had the whole thing re-assembled and working!
After all that it's now sitting nicely in the kitchen, doing what it's supposed to do. I'm pleased it's working but I've got that uneasy feeling that I've not actually done anything which should cause it to work - so it may be short-lived.