Not too sure about the blown bulbs idea though as I'd have thought that the remaining bulbs would all be at " normal " brightness.
I agree, but depending on instrument cluster design, the light from the bulbs is often spread around by light guides/diffusers, meaning that with one or two bulbs out, there is less light in total and the result looks dim (with perhaps one side slightly brighter than the other). In good designs, one bulb out is hard to notice. In the GTV/Spider it appears there are four bulbs, two in each main dial. So, maybe if there is one bulb blown in each...
Still, re-reading the original post by Chris suggests that the dimmer control is having no effect, so I agree that it's pretty likely that it's the dimmer control at fault.
Personally I would still get the instrument cluster out and see which bulbs are on - often, these will be in twist-in holders of a particular colour. Find all the remaining holders of that colour (e.g. red plastic) and check they are all working (light shines out the back of the holder, usually). Most small wedge-base bulbs are 1.2W, but they may be 1.8W or 2.3W. As I recall, Halfords has all three types.
Perhaps a poor supply or poor Earth to the Dash ?
Again, I agree with the poor-earth theory. Wiggling connectors with the cluster powered-up sometimes shows up a problem with loose terminals.
If Chris is adjusting the brightness with the correct control then I'd go along with Mitch and bypass the switch to start with.
Bypass the dimmer control, I assume you mean. In the attached diagram, it's anyone's guess what's going on with the yellow-black wire to the auxiliary instrument cluster. But it looks to me like the yellow-red wire (pin 1 of the dimmer control) should be connected to ground (purple-black, pin 3 of the dimmer control) to bring the lights to full brightness.
The ultimate problem could be a poor solder joint in the dimmer control (especially if its one of these electronic types) but I still think I'd check the bulbs first. A voltage meter across one of the bulb sockets would prove whether they're getting the full 12V, as well. If not, then I'd try bypassing the dimmer control.
And yes - for some bizarre reason, it seems that the Spider might have a roller-potentiometer (variable resistor) control where the GTV has this electronic control... I guess they had to save money somewhere on the Spider