Interestingly, Aldi are also doing a range of 20/40v battery operated tools, similar concept to the Ryobi One+ range, where everything uses the same (ish...) battery pack.
The gotcha is, some tools require 40v batteries (the big ones), others are happy with the 20v ones. But what's really quite clever - the big battery is a 20/40 dual voltage(!)
Anyway... they're currently selling for £15 for the small battery, or £30 for the big one. And, you probably guessed already, inside are 5 or 10 18650 type batteries

So there you go - £3 cheaper, big savings!