I'm not an expert on commercial leases, but from a legal aspect I would guess that, as the lease has been frustrated through no fault of your own, then you should be entitled to a refund of your deposit, rent etc plus any costs you have incurred.
I would strongly suggest visiting a solicitor regarding this as I think this probably goes above the small claims court limit. I'm not sure either about the legality of holding on to the keys as, if you are claiming that the lease if in fact invalid, there is no reason for you to have keys. Again, take legal advice.
Answerbank isn't always very accurate with the number of replies it says you have or the mails it sends out telling you that you have replies!