Farther to previous post, I have taken off the drivers' side head gasket to find a lot of water in all three bores. The gasket is intact. As there was water swilling about inside the plenum, I can only asume that the water was somehow discharging  under pressure into the breather system and being distributed into all the intakes. I am at a loss as to where this could be coming from. I examined the gasket  between the rear water channel and the alloy breather box which is bolted on at the back, which had deteriorated, but the seal on the box looked sound. The HBV is only 2000 miles old. Is there any other explanation?

Yes, and it's straight as a die.

Having thought about this, the only possible cause must be a leak past the head gasket through the seal into the breather box at the back, ever though the seal looked ok., I can't verify that for certain as I threw away the bits, before they fell into the water channels. I didn't expect the problem would be there.

Yes there a water feed to the throttle body, so it would be worth checking this for a water leak. It is only a small bore pipe so I can't see how it would have the capacity to fill all three bores, unless it is leaking under pressure, even then I would have thought the pressure would have dropped fairly quickly. I can't see how large amounts of water would enter after the pressure is released as the pipes are at the top of the engine, level with the header tank, so there would be no gravity feed.

I guessing here, that the water heating is for the same reason as many motorbikes, to stop icing of the throttle butterfly valves in cold weather?

This is a thought from when I stripped down my engine to do the head gaskets. Is is plain water or water / antifreeze mix in the bores? Are you stripping the engine down outside or under cover as we had very heavy rain on Thursday. I found out the hard way when the scuttle and the plugs are removed, rain water will enter through the plug holes into the cylinders and likewise when the heads are off, which is why I had to carefully cover the engine when I wasn't working on it with polythene to stop this.

It is of concern that you said the engine would not turnover due to water in the cylinders as a hydraulic lock like this normally bend conrods. 

Many thanks for the replies. The car is under cover, so it's not rainwater. The HBV is ok and so is the throttle butterfly housing. Interesting comment about bent conrods though. Helps in my decision whether to continue or not. This has been a pig of a job, especially the crass positioning of the coolant transfer pipe at the back.

Ideally pressure test the cooling system...

...although now the head is off...