interpet make decent heater. One thing never to scrimp on is the heater especially for a 400 litre tank (approx), thats gonna have quite a lot of fish in. I always go for the rena smartheaters - very very reliable. The fluval M series is comeing out soon - they look very good.
2 x 200w will be fine. 2x200w rena smartheater shoul be just over 40 quid.