Washed the car earlier and I've noticed a light scratch just below the rear window. It does run for about 15cm in total, but I cannot feel it if I run skin or fingernail over it - it really does look and feel like something that might polish out.

Are there any recommended polishes with a bit more "umph" to remove light scratches? Or is it worth going for something just a little more abrasive, such as Maguires Compound (or similar)?

 

This will be hand applied, hand buffed, hand polished - no machines involved.

You may wish to give this a bash. And it genuinely is a case of 'A little dab will do you'. 

 

https://www.bilthamber.com/waxes-polishes-and-clay-bars/auto-balm

I've used Autoglym paint renovator for scratches that are difficult to remove.  It has more abrasion than super resin polish but its not too aggressive. I always use a steady approach when tackling scratches, light polishing over a period of time until I'm happy with the result.