I am in the US, so I can not help you with where you could get one. However if you can find out what brands of radiator caps are available in the UK, then I may be able to cross reference this cap to another brand. Tridon is an Australian company, I do not know where they export to. I would take me a few day though before I will have time. I am out on the road at the moment, off to pick up wine from New York city.
J.R.Wadhams sells them, however I cannot tell if they are recovery type or not. However if they are not, they might be good enough to let you know what the manufacturers part number is. Then you should be able to work out a recovery type from that at a good parts store, where they not how to look stuff up in an applications book.
James.