IT WORKS! I'm very happy now, many thanks for the guidance David! 
I hooked up a POT and started some serial debugging to test and the range seems pretty good in corresponding to the bulb's brightness, but I think it seems to be weighted a little towards the ON end of the spectrum - in other words it gets pretty bright to early. How could I fine tune this?
The way it is hooked up now is with 10mA current set in the TLC (20mA works just as well) and with the 337 base pulled up to 12v through a 10k. Before I go ahead and build the 12 channels on some strip board and install the lot in my lounge ceiling I'd just like to check that this would be the preferred setup if you were making the same thing?
Pretty impressed with the TLC5940 now and the £2 3W eBay special bulbs are not too bad either 
Thanks again