PLEASE HELP we have a 14mth old gsd caled Zak, up to now he has been fab toilet trained ,very affectionate ,loyal,fairly obedient and healthy. About 2 mths ago he chewed my dads leather settee, we told him off and he knew he had been naughty but since then he's done it at home 3 times, I'm at my wits end. I know they don't like being left but this is not something he hasn't been accustomed to since we got him at 9wks. How can we stop him doing this ?

Hi and welcome to the forum.

From your post I assume he is doing this when left alone. No dogs like being left for long periods and resort to this behaviour from boredom or separation anxiety. You could try something more interesting for him to chew on. I have found the black "Kong" toy stuffed with goodies keeps my dog happy for ages and he loves to chew it even with no filling.
Also use the main search facility at the top of the page for chewing there are a couple of threads on this
Mick

Thankyou, yes he does show off and is naughty sometimes but not always. I will try that Kong toy straight away