Fred wrote
Chris Holford wrote

I've tried a plunger, and tried neutralising with spirits of salts

Where do you buy spirit of salts from? It made a brief appearance in
the tool station catalogue but I think for one edition only. I can't find
any for sale around here. It often gets recommended in this group.

Its used for cleaning brick work after laying, so a builder's supplier.

The proper name is hydrochloric acid.