Update - Received a replacement Kobo Clara HD, arrived a couple of days after contacting support. Although they said they would send out a sipping label etc and dispatch a replacement when they received the faulty one, it appears they have just decided to send a new one and not bother with the return. Maybe the returns label will turn up at some point.
Either way the new one is now all set up and working fine, the comfort light now turns down/off as expected.
Here's a photo showing the old and new side by side at with identical settings -
https://photos.app.goo.gl/nQhrJg7ZXaasD1Pg7
Really pleased with the customer service as I had expected getting a replacement would have been a nightmare, but that was definitely not the case.