• 6/8: Having spent a couple of weeks trying to source a Li-ion pack of any kind, it's time to write up some ideas. I decided to aim for 11.1V (3S) based on the peak power I'd like the amp to deliver. The next challenge was to source either a 3S pre-made pack, or individual Li-ion cells (e.g. the ubiquitous 18650s) and make up a pack. Here is a summary of my findings:
• pre-made 3S packs (made from 18650s) appear to be pretty rare, unless you are prepared to pay £50+ for a top brand
• the cheap Chinese pack I bought on eBay never arrived
• ...which may be because UK postal carriers don't carry Li-ion cells unless they are contained within a product of some kind
• making a pack requires connecting the cells either by a) soldering, b) spot-welding or c) using spring terminals/magnets
• I didn't want to waste time making a spot-welder only to find that I cooked the cell internals (see here), and likewise soldering directly to the cells. Spring terminals is an extra faff too. The best option IMO is soldering tabbed cells.
• tabbed 18650s on sale in the UK from reputable suppliers are also pretty rare, unless you're prepared to pay £5-6 or more per cell
• so the remaining option is to break open products containing 18650s and harvest the cells (with tabs already fitted of course!). The best sources for my needs were either Dyson vacuum cleaner packs (six cells, 6S1P) at ~£20 each, power banks with 8 cells (e.g. this one for £23), and aftermarket laptop batteries (6 cells, 3S2P) at £9 each.

I bought a couple of used Dyson packs, only to find they were mostly junk (new ones would have been fine as they use Sony 1900mAh cells), and a new laptop battery with a claimed 5400mAh capacity which so far looks pretty good. Will report back with measured capacity.

7/8: How disappointing: measured at only 3400mAh. Think I'll try another charge/discharge cycle and see if it improves. But at least the pack works and stays nice and cool even during quite fast charging (0.8xC). The MPPT module, on the other hand, got pretty hot even in free air. Once it's in an enclosure it'll be verging on too hot. I might change the max. charge rate from 2.8A down to 2A if I can.

9/8: After a second charge-discharge cycle the pack managed 3800mAh. Probably good enough for my needs, but I'll steer clear of Lenoge batteries in future!

25/8: Ha ha ha! The "3800mAh" pack I bought from China finally turned up today, 3 weeks late and long after I'd had the money refunded. Marked on the packet as a "$3 phone case", that's about all it's good for. I measured the capacity at a measly 860mAh. What a joke!