Didn't cost a penny - thanks to everyone here

I had a problem with one of the twins not starting - 1.9 TDi 110, turning over fine just not roaring into life.

Instantly thought of the CTS and swapped them over between the twins. No change.

Then had a play with VCDS lite with a cheapo ebay cable. On the working car I could get a response from the ECU, but not on the broken one. Starting to panic at this stage (ECU = £mega) but I could hear a faint clicking noise though, like a relay connecting and disconnecting.

Pinpointed Relay 109 as possibly being the culprit - swapped with the twin and the engine roared into life.

Took the broken relay apart and noticed an oxidised looking solder joint. Reflowed it (melted it) to recreate the bond to the PCB and now the relay works fine and all is good in the hood.

Nearly all of the information I used in that fix I gleaned over the last couple of years from the this forum - so I just wanted to say a massive thank you to everyone who contributes on here. If I'd taken it to the garage I would have had a bill of several hundred pounds! Thank you all!