"........ the soil is a lot darker with some red lumps which break up when
crushed .........."

This is the result of having bonfires on the soil, in it's self no problem.
If you are worried contact the Council from whom you are renting the
allotment and ask them to test the soil for contamination. They should do
this for you ASAP and will either give it a clean bill of health, or offer
you a new allotment.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk