Darren Clarke wrote:
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?

your problem is that you didnt remove the water from the toilet pan.
Buy yourself a little plastic siphon/pump for about one pound in a diy
shop or perhaps a car accessory shop.
Its a little plastic flexible red cylinder that you squeeze in your
hand with two pipes from it. One pipe sucks the water from the pan and
the other releases it into a container.

once you have taken all the water from the pan and you can feel the
fresh air coming up from the sewer, pour any acid descaler into the pan
and rub it all over the affected areas with a brush. Leave for ten or
fifteen minutes and then scrape all the remaining lime away using a
screwdriver or something like that. Get it all off using the acid
before you flush it out or you will be repeating the procedure.

Thats the secret. The water in the pan was diluting your chemicals
before.