Big 4 Battery

....for whatever brand you would like to fit, peruse the capacities and dimensions, and pick the most suitable. That was my approach.

Mike

... but as my ES2 (and your Big 4) has magneto ignition so does not rely on the battery I use one of the alarm batteries for £10 or so from toolstation, screwfix etc. My bike has the original electro-mechanical RB108(?) regulator.

Yes but you are 'getting away with it'. The alarm batteries do not have any way of venting any gasses due to over charging. They then dry out. If you look closely you can usually find the correct charge rate and often no more than a couple amps. Your mechanical regulator if out of adjustment can easily overcharge this battery. If you 'nip' down to the club (10 miles) and back with the lights on, you might 'get away with it' if you go for a 50 mile ride-with some 'enthusiasm' you can over charge an alarm battery.

Back to the original question, pick the biggest you can get in and the highest Ahr you can. 5Ahr I consider to be the smallest.

Amal do dry cell batteries in a black box as do Wassell (which are of course cheaper). You pays your money...