I bought a Sicce Scuba heater (Made in Italy) for my marine tank. It's slender, has a warranty, is quite cheap and seems to be a really good heater from my experience so far. Anything from Hydor is also good, although more expensive.
I situate my heater in the corner behind live rock tilted about 45 degrees. The Sicce Scuba was $30AU (or 12 Pounds) and thats the same price for 25w to 300w. Hydors range from $24AU to $79AU (or 10 to 32 pounds). Here's a link with prices. Remember that it is all in aussie dollars.
The heaters I use for my tank which is simular on size to your own are the hydor 280. Good heaters but with the added advantage that they have inbuilt water pumps to add that bit of extra flow in the tank. They come in two power strengths and flow rates. Highly recommended and never had any trouble with them. Usual LFS on net have them.