Hi Ken,
That sounds quite like it actually, 544. I did write a partial plate down in the car, 5 and 4 sound very familiar, will check it later.
Would have been about 5pmish, in that lovely barbeque weather, as you said.
I passed (indeed heading to Stranraer), in a grey S1, both sides of the road were in long queues, but moving well enough. Such was the rain that I barely recognised it as an XM until I was past and saw the back of it in the mirror, so wouldn't have had a chance to wave anyway
Its got to have been you, how many other green R reg S2s would have been on that road at the same time!
Small world...
Ciarán