If I understand correctly then this is my reply.
If a Cllr donates an item e.g. Lawn Mower (c.£200) and the Council vote to accept it then NO the Cllr doesn't need to declare an interest (by law) as they have no financial gain from doing so. However, there should be some caution.
If the Councillor is the owner of Lawn Mowers R Us for example and donates an item that the Council publicly thank him for then it could be argued that there may be some financial benefit through positive publicity. Should the lawnmower be donated because the Cllr simply wants to do something nice with their own money then this should be OK.