Don;t touch Halfords and the like for batteries - you have no idea how long the battery has been sitign there not being maintained properly. Idealy you want a specialist battery supplier - I always use http://www.cpcbatteries.co.uk/
Phone them up, tell them what you want (they do everything from watch batteries to huge great industrial things) and they will charge it with acid and charge it with juice ready for you to turn up. Obviously if you turn up with the right tools you can fit it yourself there and then.
where abouts are you - there is a place near me in enfield that you can mail order of pick up from http://www.mdsbattery.co.uk/contactus/default.asp bought from them on Ebay and realised that there were around the corner from me!!
Oh your battery is £37.98 delivered, but give them a call and see what they can do.
I live close to Chelsea Scooters, and when I needed a battery a few weeks ago I asked them to get one for me. A couple of days later they called me to say it’s been delivered and charged by them for 24h. Free installation, too, on my way to work the next morning (they open at 7.30am). They’ve got some in stock anyway, but of course, never the one you want…