Thanks for the good ideas
I like the look of that Frost product, but it sounds like it is just caustic soda, so as I have a sackful of that at work I may try a solution of that.
I also like the idea of putting the solution in the tank and then doing the cement mixer trick, I have one of those too .
If anyone can think of a reason not to use caustic soda then please let me know.
Obviously I would have to rinse it out after with water and then dry it out before rinsing with a little petrol.