I went out at lunchtime today and it cut out again! It wouldn't restart so up went the bonnet. Whilst looking at the wiring for the relays, I noticed that the main connector onto what I think is the ECU (big box in front of fuel pump relay) was sitting a bit skew. Closer inspection revealed that the securing clip was off and the one edge was lifting up. I pushed it back in and re-secured it. Now my fingers are crossed hoping that the problem is solved and it won't occur again!
It does makes sense to me as the more the connector was sliding off, the more frequent the cut-outs were, and they were definitely getting more frequent!