Not the clutch is It? I.e. loss of power, bad smell and clutch smoke not Steam? Just a thought, especially as you were going up hill and putting load on the drive train.
Thanks for your input, and your thoughts certainly do describe a possibility but it's an automatic. I know they do have "clutches" in auto boxes, so I suppose they could also slip or burn out, but I'm pretty sure it's not that because - apart from the fact that the gearbox fluid is still full, a good colour and doesn't smell bad - I had to drive it on the A21 and the M25 to take it to the garage and I could only just about get it up to 60mph with gentle acceleration and some prayers! At about 2200 rpm, everything starts happening: rapid surging/misfiring, loss of power, inability to increase speed, yet no more steam/ smoke. Kicking down is a real no-no; feels like firing a machine gun. Taking my foot off the accelerator causes the revs to drop, and the stuttering goes away. Seems very weird to me. I hope it's not a head gasket but I don't understand how it can be if there is no evidence of water in the oil? And it only happens under acceleration when above about 2200 rpm, not under less power or when idling
Actually, as I type this, a memory popped into my head from some years back. My previous 3.2 litre (3.2S) was flooded by being parked alongside the Thames in Twickenham. The water was up to over the door cills, flooded the interior and some got into the exhaust. I had to reverse it out of the flooded parking area and it misfired REALLY badly. It continued to do so, much worse than my current problem, as I drove it to a place in Essex which dryed out flood damaged cars. They did a good job and in the end there was no engine damage. Perhaps there is some water in the engine after all, though I'm damned if I know how it got in there, and especially since the problem only made itself known once I'd been using the car for about thirty minutes or so this morning, before which it had been driving absolutely perfectly.
It's beyond my ken, so I'm waiting with trepidation to find out what surprise it has in store for me now...
The garage I took it too is quite busy so I'm not expecting a rapid response.