I have a thick limescale problem that domestic cleaners aren't shifting so
I'd like to use one of the more heavy duty cleaners, but I don't know
which one to buy or how to use it.

Can I buy them in Wickes, how much do they cost, do I need to remove the
water from the bowl first, how long do I leave it soak for and do I need
to neutralise the acid before flushing? The bulk of it is under the
waterline (around the bend) but there's also a thin coating around the rim
and I think a lot is built up under it because a couple of times I've
tried using the shower head to blast hot and cold water under the rim and
this has made big chunks of limescale crack and fall off.

I imagine, like domestic cleaning products, some brands are good and
others are very poor, so which should I get? If they're expensive, would i
be just as good to pour down a cheap bottle of Happy Shopper vinegar and
leave it overnight?