If it's stuck on 50 I'd say it's the sensor but if not, as said, the thermostat is a pain to get at.
Did it on my Edition 30 a few weeks ago and thankfully it's just the one to do.  I did the sensor at the same time.  Whilst you're there and all that.  £40 for the temp sensor tho!
To get at the block mounted stat, you need to remove the alternator, belt tensioner, throttle body pipework and a few other bits and pieces.  Takes about 4 hours DIY, first attempt.  Professionals might have some shortcuts and better tools etc, so might take less time.