Thanks guys for your responses, your suggestions and ideas it is very much appreciated.
I did a test this morning, I opened the shutter of the dome just before the sun got to it.
It was -2C at 8am inside, the temperature increased to 1.5C over about 1 hour.
There was some frozen condensation on the upper wall of the dome. This was quite fine and wasn't enough to drip.
As I was sitting watching I noticed vapour going out the top of the dome through the open slit.
I had a cover over the scope and mount, so I uncovered the rear of the OTA and the counter weights to see if condensation would form on them.
I waited for about 1 hour and nothing formed.
I think what is happening, condensation is forming on the wall of the dome. When the sun warms up the dome in the morning the condensation evaporates and raises the humidity inside. There is nowhere for the warm moist air to go so it re-condenses on the cold metal surfaces of the OTA and the mount. Oddly enough there is no condensation forming on the cold metal floor.
So you guys have given me some ideas to try out. I will look at all of them and use a process of elimination to get the best result.
I am looking for the most simple fix but I am sure I will have to line the wall with insulation, cover the floor, add forced air ventilation, dehumidifier
and a desiccant bucket.
I will keep you informed of my progress.
Thanks again
Phil