I'm currently looking at the TMC tanks, because of how highly marty speaks of there equipment!
I know you stated your budget, but what do you class as medium size? In the world of marines, sizes seem to be a lot bigger, yet classed smaller than tropical. For example, 4 foot tank in freshwater would be a large tank, but for marine, most would class as a medium!
There are so many out there to choose from, and with a budget of £400, you might be better off going custom ( ND aquatics, AC Aquatics just to name two )