These fungi are growing in grassland (both mown and un-mown) near the Meteorology Department. Most of them are under lime trees but there are also some under an oak tree about 10 metres away.
In size they varied from 8 to 20 centimetres across.
Thanks to Geoffrey Kibby of the British Mycological Society we have the following identification notes: This is most likely Paxillus obscurisporus, fairly common under trees in open situations, much larger than P. involutus which is a woodland species and P. obscurisporus also has a darker, vinaceous-brown spore deposit. The other possibility is P. ammoniavirescens which also likes open areas but that species turns bright green on the cap when a drop of ammonia is applied
I have referred it to the BMS Fay: could be Paxillus involutus