Thanks very much for your replies. I am very happy to report my fish's mouth has now closed and he is eating well (actually more keenly than I had ever noted before). The day after my initial query I thought he was bound to be a goner. The top line of his 'snout' and the area running over either side of his 'scull' went all white. Imn a last ditch, I caught (which yes, involved chasing him) him and tried to shut his mouth (was easy enough to do. It didnt stay shut though but the next day was slightly better and twio days laster was shut as normal.... and now the whiteness is fading, completly off his 'snout' and half gone from around the top of his head.
So good news for me and my fish!!
There was no white fungal looking material.
I felt I could see right down his throat as it looked stuck open along with his mouth and could see no food.
All's well that ends well as they say, just hope it doesnt happen again.
I do feed flake food but also shrimp and spirilina disc's which maybe he could get a partially eaten one stuck as he my biggest fish.