Hi Marcus,
Can you describe exactly what you got from Homebase?
Was it the full Aquaroll mains kit, or did you just buy a compact float valve and make it yourself by drilling a hole in a spare Aquaroll cap?
If it was a DIY job, make sure that the float valve is positioned such that it is moving vertically once the cap is screwed on. You can check this by carefully marking the top of the cap when screwed on fully, then removing. If the valve is horizontal then it wont shut off the water.
Same can be true if the valve in the original Aquaroll kit has been set up incorrectly.
One other thing to check is that the submersible pump is not catching on the float and stopping the float from rising. Happened to me once on a pitch and the result was a small scale fountain bubbling out of the top of the aquaroll!!