My mistake - should have done more research! Big Sweetie and I visited again today, in the company of 2 other Scottish Megaraks. The stump of one of the other stones in the group is visible 5m east of the survivor. Hidden down in the turf again about 5m south lies a recumbent pillar which was probably part of the group. I have uploaded 3 photos, 2 showing the relationship of the survivor and stump, and one showing the recumbent with turf peeled back (thanks, Milgi!).
At a quick glance, one standing stone and some cairn material is all that remains of the Clach na Tiompan stone circle. Considerable damage was apparently done to the neighbouring chambered cairn when the road (on either side of which the cairn and circle sit) was driven through Glen Almond in the 19th century, and it was probably at this time that most of the circle was destroyed.
The circle was originally a four-poster, enclosing a low cairn at the centre. Excavation in 1954 revealed the stump of a second stone, and the holes belonging to the remaining two. The cairn was found to be made up of boulders, amongst which were found more than one hundred quartz pebbles,. At the centre of the cairn there was evidence of burning, with pieces of wood and charcoal. The cairn material extends to a diameter of about 6.0m, spreading beyond the edge of the circle. One of the missing stones was found lying 5.0m to the S.
Directions - Head W from Perth on the A85 for Crieff. After approximately 22.0km, and having passed through the villages of Lochty and Methven, you will reach the village of Gilmerton (2.0km before Crieff). Drive into the village, and just before leaving it again, you will see a sign-post to the right for the A822 to Dunkeld & Aberfeldy. Take this road, which climbs steeply before taking you through the Sma' Glen. After approximately 10.0km, you will see a lay-by on the right side of the road, beside the river Almond, and just before the bridge which crosses it to the right. Park here, as the track to Clach na Tiompan is a private farm track and for farmers' vehicles only. Cross the bridge on foot, and take the track to the left after it (sign-posted as a footpath). Continue along this track, passing the farms of Craignavar and Conichan along the way. About 1.5km after Conichan (and approximately 6.0km after leaving the main road), Clach na Tiompan is obvious on the left of the track, beside a track leading down towards the river, and just before the chambered cairn on the right of the main track.