Okay, we've had a good poke around inside. Not that different to a PC really.
From what I can see, the problem seems to be power related. The gameboard is equipped with 3 LEDs (D5,D6,D7), and there's one on the amp.
On startup, a beep can SOMTIMES be heard. When this beep happens, the amp LED is on, as are the two gameboard LED's (+5VDC and SOUND ACTIVITY, the third ones designates processor reset, and its normal state is OFF). The VIEW buttons on the left of the CP light up, but the monitor never kicks in, and the steering wheel doesn't initialise (it should move).
Most other times, there is no beep, and the SOUND ACTIVITY LED on the gameboard remains off (signifying a problem with the 12V line?). The VIEW buttons do not light, but now the coin mech will light up!
The tube for the marque is blown, like I said, so I can;t really use that as an indicator, but as the fan works, I would hazard a guess that the marque will be receiving power as well.
A little poking around with a multimeter showed the +5V rail running at 4.78V (I know that this is well within tolerance for a PC), but the times we tested it, the 12V rail was running at a rather concerning 10.71V, and I guess that this will fail to power the amp, hence the gameboard reporting that there is no sound activity?
So, the symptoms vary slightly, but it all seems related to power. The fuses are all okay, but there is some seemingly out-of-tolerance voltages. It is also worth noting that the PSU seems to be a refurbished (as in, there's a handwritten sticker on it saying 'spares'

) Entron black box job, so it's not the first time the PSU has had problems. PSU's seems to run for between £10-£20 on eBay, so even IF this isn't the problem, I reckon it probably wouldn't hurt to replace the unit. What do you guys think?
The good news, I've found a guy with some original NOS sideart for the cabinet! Woohoo! I'm tempted to buy 2 sets. One to use, the other to keep as spares.
My plan goes thus:
Get the cab running - probably a new PSU will fettle this.
Cosmetic work - New t-moulding, laminate repair kit (some of the edges are chipped, though not too bad) and sideart should make it a very presentable cab, with the replacement of some mismatched and manky allen bolts.
Mame it! Don't sweat, I have a plan! I Replace the rear door (it simply lifts off) with my own wood. All the PC will be attached to this with a JPac and ArcadeVGA. The control panel will be removed, along with the pedals, and a blanking plate made for the pedal hole. A new control panel can be fabricated from metal sheet, and it will use the original mounting holes, hus, the integrity of the cabinet is maintained, and it can always be put back to it's original spec without too much work.
The only thing that puts me off this is at the moment, the Jamma loom looks very intimidating! I'll have to read over Jamma before any connectors get undone, but EVERYTHING will be tagged!

So, best of both worlds then. I may even get the original sideart, and store it with the controls, gameboard and marque, and replace them with Mame jobbies. I made a 3D Mame logo, and it would be a good excuse to do some high-res renders for print!