I fitted mine myself, but even with one of those "clamp together" hole cutters, I had a devil of a job cutting one of the holes. Reason was it encroached slightly onto an area that was double skinned. I ended up drilling lots of small holes, knocking the centre out and then spent AGES with a round backed file

Then of course one has to thoroughly protect the bare metal (I only mention that in case anyone forgets

) Of course the inlet, exhaust and mounting holes have to be aligned correctly. One has to bear in mind the positioning of the heating air outlet and also allowing enough room for the gas connector and the bend in the pipe just after it. One also has to consider how much "free space" it needs around it (I guessed). When drilling big holes in your van, you need to check all that ^^^ very carefully. I checked and double checked then checked again before I started drilling !! Then there is getting the gas pipe neatly bent and fitted, including securing it correctly (there is a "standard" you are supposed to follow for securing clips every xx inches

) Then you have to source and fit a new 3 way manifold/gas-tap affair (or use a "T" piece
in the cooker circuit like I did

). Then there is identifying the (3 not 2) wires correctly and wiring them to the Stat unit. Fitting the metal flexi corrugated air inlet and exhaust outlet pipe for the burner is easy enough, as long as your holes in the floor line up

and securing it, on your back in the cold damp semi darkness, by drilling for self tappers (and protecting the bare metal) is no huge deal either. Then there is the safety angle = did I do it safely enough ? Of course, before you start you have to source all the plumbing bits you need and perhaps new metal flexi pipe / and ducting pipe (and a metal suitable hole cutter).
So I can see why some folk might be put off attempting it if this sort of work is "not their thing" (like bodywork for me is a no no

)
Not wishing to put anyone off, but if I had to do it again, and I could get it done professionally, including parts, for less that £200 I would go that route. Just me that is, your mileage may vary
