Thanks for all the ideas, I shall make a note and try out some time.
However, as I am walking out the house this evening, with the intention of going down to the boat and use some rubbing compound that I had left over from respraying my sons Beetle, my wife turns round casually and suggests that I could use limescale spray used in bathrooms!
As I'm driving to Poole I mule over the idea and purchase some with a sponge from Tesco.
Well what a brilliant idea! Poured a small amount on to a sponge and gently rub over the A frame and a couple of minutes later, sprayed off with hose. A quick shamy down and its all like new.
So there you have it Harpic 100% limescale remover, cost £1.52. Cheap and works, so pleased came home and made her a coffee .......