Ro-man do one for £77.50 (sorry its a bit more than £50, my memory was wrong)
get an electrical float switch from Varitech to cut the power when the tank is full.
At one time Ro-man had a pump that didn't like continuous running so you had to put it on a timer switch to give it a break every hour. I think that one was £50 so maybe they have changed pumps and that explains the price increase. By the way Varitech are on 01626 830830, they don't have a website.