you can get them from
http://www.bes.co.uk, they are the stainless 11" ones and are described as 'for aggressive water' part no 16656
they are 3kW and are less than a tenner, (£8.22 plus the vat) buy a few more things like ballvalves etc and orders over £40 are postage free
I would put two of them in a 15 gallon boiler, you should also use the correct rating of heat resistant three core wire. I have two in my 100 litre boiler and two in my HLT. It might be handy to buy a immersion spanner, they are only a couple of quid.
btw Stephen, I don't know whether you've tested it with a boil yet but looking at your pictures I would have the green fibre washer on the outside of the pot then the liquid cannot travel up the threads of the immersion and cause a leak. I have all four of mine on the outside and they have never leaked once.