Expired as this is now reduced even further:
REDUCED FURTHER ON CLEARANCE TO £13.99!
Seems to be good stock too - available at all 10 of my local Argos stores when posted.
I have various Guild (Argos, used to be Homebase too) and Worx branded tools which take this 18v Li-ion battery fitment and I know it is also compatible with Erbauer, Titan and I think JCB and Wickes own brand, probably others too (please check). Note: Worx call theirs 20v max or something but it's the same battery - that's marketing guff as voltage is not precise on these rechargeables.
Now £6 off the Guild branded 1.5Ah at Argos so just £13.99, which is the lowest I've ever seen this size. They claim to use high quality branded cells (Samsung) and my experience supports this as they have outlasted my Ryobi ones which cost far more. 2 year guarantee!
Always worth having a spare, charged battery on hand when you're doing a job and at this price it's worth grabbing a couple. 1.5Ah is great on a drill as it's light and drills are not too demanding. With something like a circular saw you'd be better with a larger capacity battery though this one will do the job for a few minutes if necessary.
If you know for sure any other brands which take this battery please post for the benefit of others.