Im with Spruce on this one , he twisted my arm a bit with regard to Numax , I went for the xv24mf as I wanted to use it as the van starter also , the 31 wouldnt fit under my dinky bonnet !
It has already lasted longer than the £50 cheapies I was stupidly using before , its been sat in the engine bay since 15.01.18 without a bench charge yet , but dont forget I have alternator and solar charge on the go .
Numax is money well spent if you look after it as you should , the clue is in the weight of a good battery , also the xv24mf has a listing of 500 cycles , not many cheaper ones come close .