The suggestion, from Dave K, of a lesser yellow underwing looks good. Most of the ones shown on the internet are more brown, and paler, but the wing markings are right. Unfortunately, we didn't see the underwing. Thanks, Dave. Jon
It seems to have a resemblance to the Reddish Speckled Dart Cerastis tenebrifera, however this is a north American species. The Red Chestnut (Cerastis rubricosa) is found in Scotland but not in August. Other Cerastis genera are similar, so too are Orthosia and both also variable, yet yours is especially striking in its brick red with black vein and cream patterning. The closest I have found is White Marked (Cerastis leucographa) but not completely happy due to location and colour-filled spots. Moth recorders might be interested in your sighting!
The consensus now seems to be a yellow underwing, possibly the lesser variety. Jon