I have a 2 year old labrador, who when left will chew personal items in the kitchen or destroy the back garden when left. He has another dog for company, only needs 20-60 minutes being left before he finds something to chew. He is left with indestructible toys, kongs with food inside or other toys with food. If he is chained at his kennel when left, then he digs. He is walked usually 3 times a day and is a calm dog, good in every other way. I am at the end of my tether, the only way that he will stop destroying is if I am with him constantly.
Please help, I have had nearly 2 years of this.

Why not consider investing in a dog cage?  I have one now for 2 of my Staff pups and they love it...it's their hidey hole and sleep very happily in there without any encouragement from me!  Their mum however is not destructive and is totally trustworthy when we're out so she is not caged.

If you ensure he has all his tasty treats with him in the cage, he will soon learn to recognise it as his place of sanctuary.

Good luck..