Hi
I hope this may of be so use to you. I have worked at sailing clubs for most of my working life. They have the same issue that you do - to the point where people just dissappear! The way these clubs have to do it does vary but in general you have to first try to make phone contact, then send them a letter informing that they need to pay or remove. If you get nothing from that then you need to send a registered letter to the people. If this then comes back to you then i believe that you can sell the goods but the money that is made gets put into a bank account so that if they do come back and say 'where is my van/boat' then less your costs they get their money back. I am not sure if this is the legal way but i would check with a solictor first, but this is what these clubs have done and it seems to work. Timescale point of view here where i am at the mo is fees are due in january, an audit of boats are done in march/april and then if nothing is heard then they get sold at the clubs annual diner which is in november.
I hope ths has helped - regards andy