Since the original poster seems to be in the UK, there may be a closer source for a 240v-to-12v converter/charger than BestConverter here in the US. I wouldn't know that that source IS, but I'd expect an RV repair facility to know.
1eyebri: Assuming the same brands are available to you in 240v versions, I've never heard anyone complain about Progressive Dynamics or Iota converters.
PS: I don't mean to give the impression that Randy is not a great vendor and very helpful source of information. I have a Powermax converter in my Argosy purchased from him, I'm very happy with it so far, and its 3-year warranty suggests I'll be happy with it for some time to come.