Well the commute consists of 30 mph for about 5 mins until I get to the motorway and then I'm on that for about 15 mins at 70 mph. I would've thought that would be quite favourable for warming up.
I suppose the thermostat could be stuck open but in my experience, when this happens the car takes ages to warm up and may even hit normal temperatures when going slowly but as soon as you go faster, the air flow brings the temperature down. That isn't the case here. Once it gets to 90 oC, it doesn't budge whether you're stuck in traffic or on the motorway.
Might give the garage a call. Wish I had Phil1980's car!