I finished off my head testing thingy today (see photos) and the short story is that both valves leak like sieves.
It was a bit of a surprise as I would have bet money on the exhaust being still ok as the valve face looks ok, the valve seat looks clean and even, but the air just pours out.
The intake valve was the one I replaced and under magnification the valve seat looks trashed, but it wasn't leaking much worse than the exhaust valve.
There really doesn't look like there's much meat left to have a go at re-cutting the seats, so it may be time to bin it.
Fingers crossed with the new head when it arrives..