I've been phoned quite regularly the last couple of weeks by someone claiming to be from Orange telling me i'm due an upgrade to a new free Iphone 4s due to being a valued customer (since 1994) I only spend about £20 a month currently so is this a scam ?

I would ignore anyone calling direct. Even if they're legit.
Contact your provider and ask.

I got a free iphone 4s on Orange from one of there shops. I got it on a business contract

Hi!

If you have been called direct I would double check it was an Orange operator and not a third-party firstly.

The iPhone 4s 16gb is free on £46 a month (However for a limited time it is free for £41 a month for upgrading customers)
The iPhone 4 8gb is free on £36 a month

Thanks.