Re: planning retention
the fact that it has been there more than seven years means that the council cannot take enforcement action against you to have it removed.
if you are planning to sell it it would be best to get retention on it if you can, but if it is refused the council cannot force you to remove it, and you would just sell it without the benefit of planning. (it might affect the price, but not substantially as the council can take no action against the new owners either).
For the benefit of doubt it is best to prove it is there 7 years, this can be difficult, but a lot of councils have aerial photographs of the entire county going back over the years. (very useful to prove people are lying about how long a building is there)