Agree that cillit bang is great for limescale on sinks, shower etc. You have to make sure it's the one with the pink top. As the green top is a degreaser rather than a limescale remover.
But for toilet the best thing i have found, and I'm in a hard water area too, is the tesco own brand toilet tablets. If you can leave in overnight they do a great job.
OK I had to do this last week and it is a naff job. Firstly I emptied ou the water - what I find is that if it is REALLY bad the products are too disolved in the water. Runbber gloves and a small pastic cup does the trick (I even sponge out as much as possible that I can't get by the cup) Then I use Viakal - and lots of it. Leave it overnight and then scrub. Mine was really brown and when I tried other methods I was still almost chipping off the limescale in chunks. This way worked great for me.
If all else fails, get a bottle of spirits of salts from a DIY store. Use extreme care - it's concentrated acid. Use a brush to pump most of the water out of the toilet and then, with great care and using rubber gloves and if possible eye protection, pour the s of s round the bowl. Leave a few minutes then flush. Be careful not to inhale the fumes. Keep children away.