Hi
Use harpic 100% limescale remover , £1.60 at tesco, or use their own brand which is cheaper.
let it sit for a while with some hot water and then use a toothbrush, a scourer or a screwdriver to get the really stubborn bits. If you have limescale like we do down here then you may not get it all out.
You could use acid (spirit of salts) but do this outside as the fumes are really toxic (mask, gloves please)
Martin
P.S. Where was my Easter Egg this year, I distinctively asked for a large Thorntons egg with chocs and all i got was a kitkat one with cup