So, once it's going the new engine's sweet as anything; but it suffers from the same malady as the previous one in one respect. I can't get the starter to turn it over with any vigour.
I've cleaned everything I can see (all the connections to the starter solenoid, the earth cable at the front of the engine, the battery earth). I've checked voltages where I can - and it appears to be getting +12ish when starting. The solenoid is, remarkably, working exactly as it should.
Is there another earth cable somewhere that runs to the starter? Something else I should check? Or is it my starter walking the long walk to death?