On 15/05/2021 13:02, RobH wrote:
there are limestone splash marks on the brown plastic edging I used between the path and soil.

It would be odd were it anything but limestone residue, unless there's
something else in the self-amalgamating, but as Mr Hogg says, try brick
acif. You can get 5L very reasonably from Toolstation (Bostik I think,
hang on... yes, "Bostik Concentrated Mortar & Brick Cleaner 5L" £6.15
(you can get it cheaper "Feb" or whatever elsewhere)). You'll also want
a brush or rag and rubber gloves. Do a small bit first to check it's OK,
the stuff's hydrochloric acid with additives.

If it doesn't work, you've enough really, really efficient lavatory
descaler for quite a while, you only need about 50ml at a time.