Don't piss around with them. My mate lost an eye doing that.
And an experienced yachting instructor lost several fingers.
It isn't worth messing around with them. I have a bag full of the damn things and can't find any way of disposing of them. Police don't want to know. Last time we tried to get rid of in date flares (ferry wouldn't let us on board with them, we were going to The Netherlands to pick up a boat), the coastguard wouldn't take them and the police made a huge fuss, had to get the firearms officer in especially to take them and put them in the safe. These are things you can just buy in any chandlers in the country without special permission. It's bonkers.
Whereas PROPER POLIS will just take them off you over the counter and place them in the Bomb Bin1 in the back yard for collection/ dealing with by the right folk at a later time.
1) An receptacle looking much like a red grit bin filled with sand (looking much like a red grit bin filled with sand because it is in fact, a red grit bin filled with sand but with a lock on it so therefore being secure and safe for storage of unwanted ammunition, pyrotechnics and possibly crusty auld grenades)
What was even more bonkers was the conversation when we tried to get on the ferry.
"You can't come on there with those, they are explosives."
"They are yachting flares"
"Yeah, but they are explosives."
"Well, we can understand you are concerned about safety and them being set off inappropriately. Can we hand them to a member of the crew, to the ship's purser to keep for the voyage?"
"No, the ship can't have explosive substances on board."
"Um, are you telling me that the ship doesn't carry safety flares? That the lifeboats aren't stocked with flares?"
"That's got nothing to do with it."
We gave up. I refrained from pointing out that it was carrying several hundred vehicles filled with tanks of petrol. Binned £70 worth of flares, nearly missing the ferry in the process.