Hi @pasanda yes I have an update. He was summoned home and arrived mid afternoon.
We were calm and sat him down saying it was to discuss a few issues we were concerned about. Started off with asking him to account for all the money he was given for Christmas.
He admitted immediately he'd spent a lot of it 'on the jar' and said he knew we'd found it.
We actually left it in place in his room as I thought the best approach was for him to hand it over willingly rather than us taking it away. Taking it would have caused rage and aggression- he can lash out physically when frustrated/angry.
He said he spent £220 on 1oz for personal use to last him a year 
I'm not sure that was entirely honest about lasting all year.
We had a good talk about how he's overstepped the boundaries of our home and rules as well as putting both us and himself at risk. I told him I had real concerns about his mental health and the effects of smoking this which may not show til years down the line.
I used the example someone gave upthread of the guy in a cell naked and covered in his own shit masturbating whilst suffering from cannabis psychosis (I thought he needed a bit of shock factor).
He said he's not addicted but his reaction was devastation and utter loss. He said he didn't know what he was going to do now, which is worrying. He said he feels sad a lot and doesn't know why. I think he's been using it to zone out.
We told him the jar and contents need handed over to us, he immediately jumped up and went to his room to protect it.
I made sure the doors were locked and keys removed then followed him. He pleaded to be allowed to sell it on to get his money back but was told that would make him a dealer which he couldn't quite see at this point but we explained until he understood.
We then said he either hands it over to us or we take him and the jar to the police station. (We wouldn't - but he had to be made to see the potential alternative consequences)
In the end he could see that he had made a poor choice in blowing his money like that and bringing it into our home and willingly handed it over. I felt that was a turning point as psychologically I wanted him to it make that decision.
Afterwards we had a good heart to heart, he was made to know we love and support him but have to protect him by being cruel to be kind.
He has massive worries about his future and says he's often angry with posts on social media - particularly bragging posts or things that trigger insecurities within himself. We talked about how to hide certain people/reducing time on screen etc.
I guess a lot of teens struggle with these worries but the main thing is we know and he can turn to us instead of drugs. It just shows how easily kids can get sucked in to the world of drugs nowadays and it's so easy to get hold of without being brought up in that kind of environment.
There will be some consequences, no friends are allowed in, no sleepovers and the rest of the cash is going in his bank account and I have his card. He can work towards getting these things relaxed, but for now that's how it is.
We have agreed to allow him to take up boxing if he returns to his football which he agreed as well as do something as a family once a week. It ended positively for now with no raised voices or violence.
I'm so thankful for everyone who had any genuine input into my post, it really helped me work things out in the 12 hours I had before he came home. I hope you all get through this minefield with your kids. I'm sure with love and support and a watchful eye they'll be fine and come through the other side.
Today we have sorted it, but I'm not naive enough to think that he'll not try to get more or do it again so my senses will be on high alert for the foreseeable.
The only problem I have now is a big stinky jar of £220 worth of weed sitting in my locked filing cabinet 
What a waste of money.