Thanks for the feedback Agile.
This is the tank I am considering: http://www.screwfix.com/p/cold-water-coffin-tank-25-gallon-1390-x-500-x-310mm/23238
If I connect two in parallel, does one need to be set higher than the other? Is there a kit I can get to connect them?
Our shower has very low pressure and our loft tank has not been laid properly (so it's lid does not fit). The tank has no insulation, and I hate to think what's fallen into it over the many years it's been up there.
I thought I would fix both of these problems by getting a new tank installed...