Well the Windsor cycle shop let me down, despite advertising the TX50 on their website as in stock and confirming that by telephone, they then decided they couldn't source one and cancelled my order. Won't be going to them again...
Seems these things are much rarer than I first thought. I emailed Shimano tech support to see if any other models in their range might be suitable, as I was guessing (hoping) that all Shimano shifters pull and release the cable at the same ratio, even a 8 or 9 speed shifter would theoretically do the job. No reply from them for over a week :-(
Wisper down at Southampton have now come to my aid and found one on a crashed bike so hopefully my Wisper fixie will be geared up again soon