Managed to fly it again yesterday with the re-cushioned fuel tank. I was very hopeful that this was going to be the end of my troubles but alas, the engine is still quiting.

I put a slower prop on it (11x7) and noticed that the engine became less suseptable to the surges, but it still failed on me a few times. I'm as certain as I can be that the engine is sound and the problem is fuel starvation.

I've decided to take the fuel tank and pipework off the aeroplane and start a fresh.