I had the same issue with my 2013 one.
The glow plug relay is on the far right hand side as you look under the bonnet. You may have to take out the headlight to get at it.
Test each glow plug via the relay plug, they should all be about the same resistance. If not, it’s probably that plug. Mine were all about 1.2Ohms and the busted one was 0.6Ohms.
The one on mine was number 1, the only one you can remove without taking off the inlet manifold, so I just changed that, then another went a few months later.
This is the thread about mine from the motorhome forum.
https://www.motorhomefun.co.uk/forum/threads/glow-plug-warning-light-resolved.304743/#post-6378668
Whatever you do, if it won’t move, don’t snap it off. I used a torque wrench in reverse set at 10nm, they all moved but one in particular took 10 mins of gentle back and forwards with lots of lube.