My first thought was to search on Google - which returned a bunch of hits on weedy 7ah batteries, all made by folks I'd never heard of, and at around thirty-odd quid or so. I wanted something bigger ................. much bigger.
I spent a pleasant hour searching on battery case size (that's a trick to see performed!

I settled on a company called "Tayna Electrical Supplies" http://www.tayna.co.uk/ who were able to supply me with a Varta (Good quality make) 514 011, 12V, 14Ah Battery for just £23.99 (+ electrolyte & postage). My battery arrived, as promised, the following day, took a 12hr charge - and the starter now spins the motor with unprecedented vigour!!
The difference is quite marked, especially if trying to start after a week or so. Gone is the languid and sluggish labouring of the 7ah battery and the rapid draining of the meagre 7ah jobbie - the starter now has plenty of power to draw on to spin over that big single ...........
The dimensions of the new battery are
Length 135 mm -
Width 90 mm -
Height 167 mm.
Slighty thicker and taller than the original, the replacement fits snugly into the exisiting area, requiring a minor mod to the rubber securing band to encompass the slighty fatter body. I simply screwed a curved 1" plastic block onto the existing hook-on point to extend it out past the case.
A recommended upgrade next time your 7ah battery dies.
