As before, just cleaning the trap etc. I do once in a while use it for cleaning steel etc but not very often. Just wondered if there was a cheaper source than B&Q.
For descaling a drain you could use the more common HCl in the form of brick/patio cleaner wheich can also be used for de-rusting (see other threads as it has been well covered). If you are in a hard water area and occasionally de-scale the kettle you can re-use that as it is going down the drain anyway, just tends to be more benign.