hi Dave,
I should update on where I got to, which would help answer your question, and hi from a fellow Brit
I was intended to buy the correct UV for my skimmer, but my order got cancelled about 3 weeks after I ordered it. So in the end i went for an inline Jebao UV light, this was working ok, but then i realized it wasn't enough, I guess the flow rate over the UV was too high, I am using a 3600gph pump and 2in PVC pipe. So I bought a second UV light, exactly the same, and modified the housing. It was easy and is exactly as you say, you remove the black plastic outer housing, and inside is a glass tube that screws to the light fitting making a watertight connection. I attached very simply to one of the filter housing using a cable tie, and it works great. I haven't any pictures as everything is packed away for winter, but I am using the 2 lights together (probably a big no no, but I honestly don't know if the second is working at all)
as for size, I can't remember the conversion to UK gallons, but my pond is 6ft x 12ft, 2ft deep around the outside and about 3.5ft deep in the middle.
i don't really have any problems with this set up on that the lights are cheap and as such behave like cheap UV lights, sometimes they turn off, so I have readjust the bulb in the holder, but i check and clean the skimmer every 2-3 days so I see catch it early.
this is the set up i bought
http://www.ebay.com/itm/JEBAO-36-WA...721?pt=LH_DefaultDomain_0&hash=item4ab310b9d9 unfortunatley not available in the UK, but if you are interested in doing this yourself, there will probably be something similar.