Thanks for all of the excellent ideas and posts on this topic.
At the w/e I was able to get up the loft and look around more.
I discovered that there were only three places that hot air could get into the loft.
1) A soil vent from the ensuite loo. This was covered over with insulation and so I don't think there's too much of a problem with it. I have to vent the soil pipe into there anyway I think? However in the same soil stack there is our utility room with nasty condenser tumble dryer. There is a gap in the boxing down there that would allow hot wet air to go upwards. Whether hot air can come up all the way up to the loft I don't know.
2) The airing cupboard (with lagged hot water tank) had 6 holes in the ceiling all a fairly sloppy fit. I have now stuck loads of silicone sealant around them to make them airtight (or near enough).
3) The loft hatch on the landing. Just a piece of wood that you push up to get out of a recess. I am looking to replace this with the PVC insulated door that screwfix sell. Unfortunately this is next to a wall and so the coving gets in the way of having a full frame around. Head scratch time.
While in the loft I cleared out the airbricks at each end (they were pretty open anyway). These are 17.5x17.5cm each. In the eaves there are small round vents spaced about 35cm apart. There are no ridge vents.
Insulation is about 100mm deep all through.
The two cold water tanks are lagged but the lagging had fallen off the central heating tank. None of the pipes felt warm so I don't think there are plumbing problems causing this.
The loft is very cold, it's certainly not a sub tropical warm area at all!
The water was virtually streaming down the inside of the plastic lining. Every few seconds it drips down.
In the short term I've emptied anything out of the loft that had fabric or wood and I am storing it in the (relatively) dry garage. The missus was in tears as some of our baby storage stuff (toys, moses basket etc) have mould/mildew on them.
The plan of action is: - Silicone seal up any small holes (DONE) - Keep a window permanently open in the utility room for the tumble dryer (DONE) - Keep bathroom/ensuite windows open on the latch (DONE) - Change loft hatch to insulated type - Add humidstat extractor to utility room - Add humidstat extractor to bathroom
Stuff I'm still not sure on: - More insulation? I don't see how this helps without doing the other things? - More eaves or ridge vents? - Active extractor fan in the loft venting out through an air brick?
Once again thanks for all of the posts.
Painters10