Simon
Definitely CTEK, as above advice. I would recommend the MXS 7 or MXS 10, as they have the useful ability of being used as a supply, if set up correctly initially. This avoids the chance of loosing electrical current and therefore numerous settings, if you ever have to change the battery or disconnect the battery for some reason.
I have a CTEK MXS 7 and it keeps my S-Type (renown battery drainers) well up together and always serviceable. A very worthwhile investment.
Best Wishes and Regards, John