I agree with Emrys. Poisonous herbicides like Roundup are not suitable
for use where food plants are to be grown.

Alan--do you ever think about what happens to "commercial" potatoes plants
before the crop is harvested.

I assume you are saying that commercial potato growers use glyphosate
on the field before they harvest the potato crop.

If they do does that make it good practice? I would have thought that
if someone is taking the time and trouble to maintain an allotment,
then they are probably aspiring to growing food that is not tainted by
commercial practices.

I dont want to put words into the OP's mouth, just that would be
uppermost in my mind if I had the time to 'dig'.