Problem looks like it's fixed.
I'll still have to recharge the desiccant plug again, but fingers crossed.
The first shot is 1 min @ 100% cooling immediately after it stabilized at -14°C. No frosting. There is only an outer ring of frosting present. The second shot is from 5 min later and the frost has retreated considerably in that time.
Well on the way to being a very happy chappy.
Once the new seal gets here I'll bake the desiccant again, while that is happening I'll replace the seal and dust the inner surface of the faceplate glass window (might replace the seal at work in the airconditioning
).