OK. New inlet pipe fitted. The misfire that was intermittent has returned and is now much more persistent. It was misfiring when I started it after fitting the inlet at Alfa Workshop, so Jamie very kindly had a look. He suggested plugs or coil packs, as the plugs are pretty old.
Today I've taken all the plugs out and found oil on both plugs in cyl 1 and the small plug in cyl 2. Its very clean oil, so I think it is spillage from topping up, rather than indicative of another problem. The plugs on cyl 1 and 4 were a bit sooty, which says they're running rich. That correlates with the 1.23 fuel correction that MES was showing on bank 1. So, cleaned the plugs as best I could and put them all back. Got the oil off the plug caps too. Remembered to reconnect the flying earth lead on the right of the coils.
Started it up, and the misfire is still there once the idle settles. It doesn't misfire with some revs on and pulls well otherwise. Its just at idle that it jumps around, although subjectively I think it is better than it was. I expected that it might take a while to settle properly, so took it for a drive, giving it some gas occasionally and getting the revs up.
MES showed bank 1 precat lambda making in some cases 2.0v, which is way out of its usual range! It did settle as low as 0.001v. However the switching is slow so I think the misfire and its attendant overfuelling have messed the sensor up. When its cooled down I might pull the sensor and have a look, but you can't tell much from visual inspection.
I think next stop on the odyssey will be a set of plugs. They're easy enough so I might do them myself.