We'd seen quite a few Parasol Macrolepiota procera still putting on a show and at first we thought this was another one of those. But the white, slender stem gave it away as something different...
... I believe it's the Slender Parasol Macrolepiota mastoidea which I've seen only once before, on a field course in Suffolk with Geoffrey Kibby.
It gets its scientific name from its resemblance to, yup, you've guessed it...
For the record
Date: 5 November 2017
Location: Near Spithandle Lane, Ashurst, West Sussex
Grid reference: TQ174140
Record entered into FRDBI 07/09/2018