New set of big end bearing shells and it's all better. One of the shells was in a right state, it had worn though the first layer of metal to a layer of copper, and then worn through that. oooopps.
Apparently the oil was well past its sell by date and the engine was a bit gunked up. But hopefully a few oil flushes over the next month and a few hard blats and it'll all be cleaned out.
Anyways, only a £250 bill so was a bit better than I expected. And I can definately recommend Millway Vehicle Specialists in Andover, top rate stuff!