Thanks PinkMouse, the coupler are Masokits, they are slightly fiddly to make up and need a bit of speed with the soldering iron (I have several solid sets where I lingered too long)but they work well and are quite robust. You also need a source of supply for Brass pins but I just Googled them and found some that would do the job and bought enough to probably last my life time. They also make quite good T handles if you file them down a bit.
Jonathan, thanks for the info on the Cordon Gas tanks, I hadn't realised the NE had had some. I seem to recall someone doing a kit for them - CSP Model marketing list one (Falcon used to list one as well I believe, unless it's the same one). Another one to add to the list, I have a soft spot for gas tank wagons.