If you've got a certain type of Clayton Heater (I believe supplied for a short time only in 1996/97) it has a 13mm connection, with the usual 16mm connection at the valve end.
This means either getting hold of the OE part from Clayton or using a reducer connection and different sized pipes/elbows.
I've actually just done this job myself and, having previously obtained a variety of silicon and rubber pipes and reducers, I ended up getting the OE pipes from Clayton in the end. This was because I found there is not really enough space between the heater, bulkhead, and valve to do it sensibly with "join-ups".
FYI Clayton part no. is CX4609. You need two of these and cut one down for the shorter elbow (they've stopped making the shorter one).
Do a search and there's a lot more on this.
Having said all this, I hope you have the standard heater, as it seems this is a lot easier all round and gives you more options (eg for using silicon).
Peter Burt