John,
I don't ever recall seeing Millfield listed in Pooleys or the VFR guide? I've certainly seen powered aircraft flying (low) around the site but these could well be tug aircraft?
The number for the Borders Gliding club is - 01668 216284
There is an airfield about 6 miles west of Berwick called Winfield, despite flying in that area a few times I've never actually spotted it from the air??!! It's situated just off the B6461 (Kelso - Berwick road). I can always eyeball Charterhall without too much trouble but Winfield is a different matter.
If you are ever driving around there, look out for the electricity pylon with the red light on top, Winfield is approx a mile on the right hand side - assuming your heading towards kelso.