Hi Kev
I had Mk 2s do this to me on a number of occasions when road testing vehicles that had been in storage for long periods. The engine would start, run well with bags of power then drop off and fail, but start up after a short period. The main culprits were the choke casing seals, which had dried and shrunk allowing air to pass. Once the engine was up to temp the main jet could not cope with the constant air bleeding into the mixture. The other contenders are the fuel pumps. There are as you know two independent units. But they are linked to each other and each carburettor, so that if one fails the other will supply fuel. The one that did this to me had a pump with a crack across the valve face, that rendered it unable to move fuel. The other pump did the work but had a valve sticking issue once it got warm.