what nature of ‘permissive’? Informal? DEFRA countryside stewardship? Regardless, as a general rule permission being just what it means so could well be suspended at any time.
No direct legal duties, other than standard duties under firearms acts etc. (eg intent to cause fear of violence) – common sense, respect and Wheaton’s law applies (to both sides)
Can only reiterate what we said under the IMBA banner years ago:
What should you do if you encounter a pheasant shoot when out for a ride on a bridleway or byway?
IMBA recommends that you wait for the “drive” to be completed. This may take up to 15 minutes. You may also find that the “beaters” are positioning themselves for the next “drive” – if so, we again recommend waiting for the drive to be complete so avoid disturbing the birds prematurely.
While you may feel that shooting pheasants or other game is rather an odd way to spend the day – remember that the shooters and beaters will quite probably have similar views about mountain biking!
BASC advice here: https://basc.org.uk/wp-content/plugins/download-monitor/download.php?id=49