In message , dirt dibbler
writes
Having taken on an overgrown allotment this season, i have been fighting
back a forest of weeds all year.
The problem seems to have got out of control over the last month & i now
have a wall-to-wall carpet of weeds.
I feel like blitzing the whole lot with weedkiller in the autumn, can anyone
offer advice on what the best time
of year would be to do this & possibly the best chemical to use?

You can hit it now with glyphosate to clear the ground. Wait a couple of
weeks (or maybe 3) until it is all tinder dry then make a firebreak
around the edges and torch it. That is by far the fastest way to clear
severely overgrown land. Most seed heads will be destroyed too. As will
your fences and garden sheds if the firebreak is inadequate.

You must hit the weeds in active growth and you do not want any of them
to set seed. Especially not the thistles which are almost in season.

Regards,
--
Martin Brown