ESB wrote:

I was clearing up my mother's garden shed and noticed a lot of bottles
and packets of what are now considered to be illegal products (for
example, the original Jeyes Fluid, Cutlass, and an old B&Q weed and
feed)

Depending on the chemicals you might like to look up the MSDS for the
ingredients. Some things could be used (out of licence) but wear eye
protection (several formulations were made illegal because morons used
them as eye wash or drank them to commit suicide). Unless it is
something really nasty and/or carcinogenic the least environmentally
damaging solution is to use it up according to the instructions.

Legally you should take it for safe disposal. Most local authorities
provide something for toxic materials at their waste disposal sites.

Stored carelessly they may well be deactivated by hydrolysis anyway.

Does this make her a criminal and if so, can we expect a raid from the
'Plant Police' ?

The scariest things I ever encountered doing this sort of thing we

An evil looking corroded container in my late uncles garage that turned
out to contain mega concentrated washing up liquid (sans dye).

An innocent looking but aging and brittle 5L plastic pink parafin
container in a greenhouse that on careful inspection turned out to be
full of 35% HF ("glass cleaner"). I put it down again very very
carefully. Had it ruptured I would be dead. Specialist disposal
required. The HF *safety* film is notorious for causing lost time
accidents. *

* AFAIK HF is not offered for sale to the public in the UK.
(but it was to grape growers in Belgium)

Regards,
Martin Brown