Sean I agree here is my considerations.
Cost well thanks to a good friend of mine I have the tank and possibly a few bits and bobs so we are on target to come in at £100.
I believe nothing can stop any one fitting one providing once completed it is certified and added to the LPG register.
We have a man who has come highly recommended who is familiar with tight wads doing there own work who providing its all done to the correct standard will test and do the paper work.
My big consideration is how it will look, so I Have thought of hiding the tank in a belly box, fuel tank etc, I can put the controls out of sight so at a casual glance it should look ok.