This happened to my parents when they sold their house last year. They bought it in 2002 and there was nothing shown up in the searches conducted then but, as is usual in these situations, it came up as they wanted to sell.
Their conveyancing solicitor organised an indemnity policy (about �150, as I recall) to satisfy the purchasers.
No certificate of lawful development was needed. However, give your local Planning Department a ring to put your mind at rest. They are usually very helpful on these matters.