| Motobatt seem well regarded as a value for money gel battery, sold by M&P amongst others. Will be one I'll consider when the current Westco AGM item finally expires (tho that could be a while yet as it just seems to keep on keeping on).
At the other extreme I had a 18ah lighweight Shorai in my Hyper, the cost of which I justified to myself by the 2.5kg+(?) weight saving from a location high up in the tail. None available in the UK when I was looking 4 years ago so I had one sent over from the States by a mate. Cheaper than buying in the UK but still not inexpensive - but I reckon it's good value as it's a superb bit of kit.
You seem to need a dedicated charger (not a trickle charger) but that's not really an issue as it's happy to hibernate for five months over the winter without a top up. Once you've woken it up by putting a light load on it for a minute or so it's good to go. | 
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