Yaaaaaaaayyyyyyy - I've found the problem and fixed it.
It was as I, and Andrew P, suspected... a faulty receiver module. I took half of the car apart to find the CCU as I couldn't find a decent description of where to find it and pretty much knackered most of the interior trim in the process. I'm gonna let everyone else know how to get to the unit in a roadster so that they don't make the same mistake!! I will post pictures tomorrow in case anyone is interested and also to help anyone else who may encounter this problem. In the mean time however, the unit is actually really easy to get to and remove.
This is for a RHD 2000 TT Roadster:
standing at the rear of the car the unit is located on the right; behind the drivers seat. I found the best way to get to it was to remove the spare wheel cover and then the centre panel in the boot that touches the spare wheel cover. It is box shaped with an open front and is the furthest panel away from you in the boot as you stand at the back of the car:
Once you have removed the boot box you will be able to see the audio amplifer facing you on the right hand side (you will probably have to crawl into the boot to see this!).You will be able to give yourself some more light by pushing the inspection panel out into the area below the rear window and you may even be able to remove the CCU from above although I didn't try this:
Directly opposite the amp is the CCU:
It has five connectors in the side nearest you as you are in the boot. BEFORE REMOVING THE CCU, MAKE SURE TO DISCONNECT THE BATTERY POSITIVE SO THAT AS YOU PULL THE BOARD OUT, NOTHING SHORTS ON THE BODYWORK. The CCU PCB just slides straight out of its mount by pulling the PCB towards the left hand side of the car. Once the PCB is out of its mount you will need to remove the connectors by pressing their release clips and pulling sharply:
I found that there was some small corrosion on the PCB and connectors where there must be a slight leak; I can feel that the parcel shelf (if that's what it's called in a convertible?) is slightly damp. After cleaning up the connectors with some IPA and a fibre-tipped pen and also having to make a small repair to the PCB (after some serious cleaning as you'll notice the state of the receiver module above!) the unit worked!
Hope this helps someone else - I've just gotta fix that damned leak now!
Cheers
Rob