I've had a busy time since I last posted. Having visited Kent in the East and Saltash (nearly Cornwall) in the West, with various stops in between, over the space of seven days. Anyway, enough of excuses.
I'm afraid that nobody has hit the right answer yet. As I have indicated, it was a very complicated fault. I discovered the cause almost by accident when investigating one of the suggestions mentioned above.
In between the car failing the MOT and my finding the cause of the smoke, I removed the cylinder head and had it converted to run unleaded. Chris had wanted to have the head converted, but originally we had decided to postpone that until the engine had been bedded back in. However, the smoke problem combined with an increase in the oil seeping out the back of the head gasket made us think again. The smoke might just have been caused by the gasket failing. Usually when a head gasket fails the combustion gases get into the water and cause rapid overheating.
I once had a problem with a Ford engine that was burning oil and it turned out that the failing gasket was acting like a one-way valve: On the induction stroke the gasket would allow oil to be sucked into the combustion chamber, but on the compression stroke the gases would get in between the layers of the gasket and cause it to expand and seal itself. So never rule out anything until you have thoroughly investigated it.
On Chris's engine there were signs of combustion gases starting to seep under the gasket, as well as signs of the oil seeping out of the back so it was clearly time to replace it. However, there was no sign of any water getting into the combustion chamber. Unfortunately, I can't find the photo that I took of the gasket, but here is one of top of the block after I had removed the head, gasket and studs:20151205_111319
by Alistair Courtney
, on Flickr
One comment of note is that the pistons were wet with fuel. The studs were also showing signs of aging, with rust on a couple and soot on a couple more.
Chris's engine is an MED built 1380cc engine with larger inlet valves, a gas flowed head and nice set of roller rockers - as seen after refitting:20151205_152419
by Alistair Courtney
, on Flickr
One thought was that the smoke might have been due to the head becoming porous where the inlet ports had been opened out. So we figured that the first thing to do would be to have the head pressure tested.
I took the head to Rob Walker engineering just down the road at New Yatt, near Witney - http://www.robwalkerengineering.co.uk/
Rob used to run the Oselli machine shop back in the days when they were one of the leading A-series tuners and is extremely helpful. He won't suggest that you have work done if you don't need it and he was very pleased to report back to me that the head was leak tight. He also reported that the head face was perfectly flat, the valve guides were in good shape and the exhaust valves fitted were of a hard enough grade so that only work required for the unleaded conversion would be fitting hardened valve seats. After a quick chat with Chris we also got Rob to put the head through his chemical washer so that it was thoroughly clean when I came to put it back on.
So whilst Rob got to work on the head, I ordered a new head gasket and a set of uprated head studs.