Hobbyking have just announced that they'll soon be offering lighter batteries. They'll be 6s (which is about 25 volts, fully charged) and 12 amp-hours. So I'd use two of these to power the bike, giving me 50v at 12ah. We don't know the price yet, but HK are usually very cheap.
HK are expecting these to be about half the weight of existing batteries.The drawback, is that they can only run at 2C, which means you can only pull 24 amps from them. That might be a bit limiting for model aircraft, but my bike is rated at 5 amps (because it's 250 watts, and at 50v, that's 5 amps), and if it pulls 20 amps then I'm going up a very steep hill without much pedalling assistance. So I'm happy with the 2C rating
Right now, I'm using 5ah 4s batteries that weigh 528 gm. I use three of them in series to power the bike, and I put three triples in the pannier, giving me 15 ah at 50v. Two triples are enough for the whole morning out (the third one is there because I'd rather have an extra spare), and when I get back to the car, I swap the used ones for fresh batteries for the afternoon ride. So two triples are about 3.2 kg = 7 lb
With these new batteries, I'll use two 12ah 6s batteries to give me 50v, and it'll weigh around 3 lb, if HK's statement is right. That should last the whole morning, with some to spare!
So - I was thinking of getting a couple more of those 5ah 4s batteries (£16 each). Instead, I'll hold off and see what these new batteries look like,