Looks like you're wanting a response from someone in the UK. Until then, here's what I use in Canada over the winter months (it works well) with my Emira: the C-TEK MXS Battery Charger, SKU: 40-206.
I found it for sale on the UK eBay site and suspect you'll find it elsewhere in your area. On UK eBay, search for "CTEK MXS 5.0 Car Bike Battery Charger Suitable For 1.2-110Ah". There look to be more expensive ones made by C-TEK, but this one should do the trick. You want it to "maintain" your battery: kick in only when needed and the display gives you up-to-date info on what it's doing.
I received a Lotus battery charger with my Emira, but don't use it. I use this same C-TEK charger with all my other vehicles with 12V batteries that I park for the winter and have been for over 10 years, w/o any issues, so I just continued using it with the Emira.
Or your other option is to get the Lotus OEM charger, from a Lotus dealer. That's usually more costly.
I recall seeing a posting on the Forum, with UK Costco selling a C-TEK charger. Good luck!