I received an email from a supplier of bulbs in Germany after asking for info through his web page.
He said that these bulbs should drop straight into an OHP without issue. Bonus!
His email address bounced when replying, so I'm not sure I'll buy from him.
I called a supplier in the UK and the guy on the phone was very interested and helpful.
This is the supplier of the bulbs I mentioned earlier.
Sadly the 400W is mislabelled and not UV stopping but he may have others worth looking at.
He is getting back to me on the cost of a Ballast, I will make more enquiries.
I have given him the address of this thread, so if you are reading, thanks Steven
I may end up getting the 250W version as it is quite tough to source anything fitting the bill, 400W kit being hardest of all. I'll see what Steven can come up with.