I really like the Tetra HT10 submersible heater. It's small. The draw back is that it's preset to 78 degrees and has no adjustment. Since that's a good tropical temp it works great for me. I've used them for a number of years and never had one fail on me yet. I currently have one in my 6 gallon.
probably the most discreet would be the 50w BiOrb Intelligent Heater, but it's £60, then there's the Aquael Plastic Neoheater 25w for £30, or there's the Smart Heater from Rena £20
Thanks! Sadly the Tetra HT10 isn't easy to get hold of in the UK. The Aquael Neoheater looks very interesting, it's 16.5cm long which isn't bad and 'super flat' - I like the sound of that!
I was intending to avoid Rena as I've not heard good things about their reliability.