Stephen presented the following explanation :
I was advised (on uk diy) that a leisure battery might discharge if
left unused for some time and I was told that a solar charger would
top-up the charge and prevent this. What size solar panel would I
need? Does the output of the panel have to be a certain ration of the
capacity of the battery? If so, what's the magic formula?

There is no magic formula. It needs to be large enough to more than
cover the self discharge, plus any loads such as alarms multiplied by 2
or maybe even 3 during the shorter and darker days of winter.

Likely a 5w or better unit would take care of the self discharge of a
110amp/hour leisure battery, but don't expect it to recharge the
battery from flat.

Cheaper and arguably better would be the use of an intelligent charger'
at least every month or two, assuming the battery has no loads to
support between charges. Aldi/Lidl have had these chargers on special
offer several times for around £10 and they really are well worth
buying and highly recommended. With one of these you can either top the
charge up regularly, or simply leave it connected to the charger for
the entire winter.

Getting back to the solar charger...

www.maplin.co.uk sell a variety of these, but if you get one make sure
it is a controlled output type rather than one which simply keeps on
charging irrespective of need. Over charging is as damaging as under
charging, or leaving a battery to go flat.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk