We regularly (every 3 months or so) buy up a shelf of the cheapest vinegar in the supermarket (something like 29p per half litre) and pour several litres of it down the toilet, pump it into the hose, close the seacock before it all runs out and let it sit over night. That got rid of some pretty severe calcium deposits with little effort. Only works before the thing is totally blocked though.
Beware of strong acids (supermarket drain unblocker) if you have metal seacocks - some types of acid will rapidly dezincify them. There was a discussion on which acids are best on YBW some time ago, but I've fitted plastic seacocks so that's not a concern for us. The Twist & Lock pump and seals are fine with vinegar.