Pleased to see that you have had a productive afternoon Carnut! It is indeed based on an AC Ace, so you now have two of the three vital components to be awarded a point.
So, remaining locked to you, what is the make of this car (one which I have known of for years incidentially, as it has been featured in a number of magazines globally for the past 30+ years, and has also appeared at Goodwood; hence me thinking it would be easy).