Plastor food grade tanks are a bit expensive but i have not been able to find anybody else that sells anything like them.
I decided against Titan water storage tanks, because of the size of them, Plastor tanks are smaller and hold more gallons compared to a Titan water storage tank.
They have Titan tanks in my local B&Q and the 50G tank is very big, Titan tanks angle in a lot at the bottom.
Circular Titan tanks hold more for there size, but they are also quite large for the amount of water they hold.
Discounted heating gives you the size and gallonage of Titan tanks, but the second link has the same tanks much cheaper but without as much information.
http://www.discountedheating.co.uk/shop ... Tanks.html
http://www.bhl.co.uk/default.asp?t=735 (keep scrolling down the list, the rectangular and circular tanks are at least halfway down).