Sirs,
As I think I've said before somewhere the steering on the new tregs seems to feel considerably lighter, very possibly down to a geometer's change to improve tyre wear. This I think is why any small out of balance is easy to detect through the steering wheel. My own treg is the same age as yours with I believe about the same spec. 19" terra alloys and 275 Bridgestone tyres. I too feel that at times I notice a slight out of balance problem. In my case its not enough to worry me, but may be when I drive 500 miles to Goodwood in a few weeks I'll change my mind. The problem is no doubt made worse by the sheer size of the wheel and tyre which makes it much more difficult to balance properly.
I go back a few years to when it took the wheels on my Porsche 944 balanced four times with different reputable tyre depots before they finally got it right. The ones that got it wrong blamed all sorts of things on the car. The balancing is only as good as the set up of the machine and I'm sure many of them are far from perfect. Quite often they make the wheel worse rather than better.
So the moral of the story is, don't give up easily. You will probably find if you get the wheels checked by another machine before removing the weighs it will say the same as the first check you had done, out a mile. Only when you get a second confirmation of balance would I start to look further.
When you see them on the balance machine, also check for kerbing damage to the tyre as such a low profile tyre on a heavy vehicle can easily be damaged on a kerb or other sharp object.