Following on from the water butt from Tesco - Asda are now following suit. Got an email this afternoon and this was on it. Not too bad for the price even with the £5.95 postage (free delivery to a store for collection on orders over £25). Cost for water butt in homebase, b&Q etc are generally £25-£30 with an extra £10 for the stand.
You will need to buy a connector kit to plug into your downpipe to collect rainwater, unless you want to fill it with a hosepipe.
Don't forget 2.5% cashback with quidco or topcashback.