Andrew wrote:

There should be a special plastic fitting that reaches down to the
'stop cock' which is turned 90 degrees from full on to full off.

Mine has a 6-sided end, like an allen key, and about 10 mm
further up the shaft are two rectangular 'shoulders'

A "normal" stopcock key fits a plastic stopcock here

https://screwfix.com/p/stopcock-key/71652