I had almost exactly this problem after a lodger left. I took the advice of the Citizens Advice Bureau, who said that if all attempts to contact the owner failed, I could sell anything which had a value, and give away the rest. You might be expected to hang onto the proceeds for no more than 2 years and be ready to hand over the value of what you sold. But after two years without contact you can consider the items abandoned. Actually, if the owner does turn up, you can tell him you will charge him rent on the garage space. Best advice - send message/email/letter to last known address, send ultimatum of 6 weeks, then dispose of stuff.