Just looking at heaters at T7 and they have just acquired a 3d printer. They are willing to make pipe reducers to your own specs, in case you can't find the one you want
http://www.t7design.co.uk/
I have one of their heaters in my car, very Impressed.
interesting, which one did you get?
3.5 kw with the 4 outlets, two for the windscreen and one each footwell, easy to install and it replaced a mini clubman unit which was massive and not very effective
We did negotiate a discount with T7 some time ago so worth mentioning you are a BKCC member if anyone is intending buying one 8)