Hello Steve,
You are correct it is a Juniper Shieldbug. It has become very widespread in southern England (south of The Wash) since adapting to feed on several Juniper related species like Lawsons Cypress. It's still confined to wild Juniper in the north-eastern counties.
If you have a look around the area where you found it, I'm pretty sure you will locate it's modern foodplant in the immediate vicinity. The points of ID are visible in your photograph, the reddish boomerang shaped forewing markings, the basal segment of the antennae not extending beyond the head and the two segmented feet.
Harry
Cassanova and Drunk of this Parish. :flowers: B