A quick update. In consultation with the RR man, his recommendation was to go for 32mm chokes which I ordered and they arrived today. I could only get one of the chokes out by hand, so I bit the bullet and took the carbs off to be able to tap the chokes out which I did. I fitted the new chokes and in doing so found one the aux venturis wouldn't go in as far as the others by 1-2mm. After some amount of taking out, looking, head scratching, I compared it with the others and its overall length is greater than the other 3 by about 1.5mm. The normal perfectionist in me wants 4 identical ones, but if I order a replacement, I don't know whether it will be any better. So I have put it all together and the engine starts and runs well enough for it to go on the trailer and off at the other end for the next RR session. I also had some fun with the trumpet/backing plate studs, but I'll update the other thread. And Andrew thanks for the reference suggestion, most kind.