This is probably covered by the torts Interference with goods act 1977. After 3 years it would not be unreasonable to consider the goods to be abandoned, you have a duty of care whilst the goods are in your possession and you may be held liable for any damage while they are in your care.
Do as coccinelle suggests or if you wish to act strictly within the law make a reasonable effort to find the current address and make a record of the efforts you have made asking friends etc, write to the last known address or publish in a local newspaper and say you will be disposing of the goods in one month if you are not contacted and give your telephone number
(a month is a reasonable time and English law likes reasonableness). If you hear nothing dispose of the goods but remember if you sell the goods the money is not yours, retain it for return to the owner for the next 6 years, do not make any charge for the storage .