The altitude of the cloud base would have the biggest effect I suppose. Higher base = more chance to be lit from below.But not necessarily around sunset... not sure how much prior to sunset you can get this effect, though.
The cloud cover in the sun's direction might play a role, too. Clear sky all the way vs overcast sky to the horizon that blocks incoming rays.
Local topography is another thing. The sun might disappear earlier altogether behind the mountains of the Highlands to the north, although I reckon the cloud base over the Pitlochry area would need to be quite low to be influenced by this.