chris French wrote:

Buying some citric acid powder from a chemist, or hardware shop is
probably cheapest

Under the NuLabr rule of terror, asking for citric acid sees one branded
as a drug dealer.

The correct acid to use for descaling is sulphamic acid. This has the
advantage that it's impossible to make a strong solution of sulphamic
acid because it's not very soluble.

You can buy it he http://www.mistralni.co.uk/ for £14 a Kg, and a
kilo will last a good long time.