I do not think it is reasonable. He can just get a key cut. He doesn't need to spend £2,000.
As such you do not need to pay it and you can claim your deposit back (less a key cut).
You need to write and set out your losses and request a refund within 14 days or say you will go to Court within 14 days. You should make sure you send this signed delivery and keep a copy.
If they do not refund you then you can issue proceedings in the County Court. You can either do this online at: www.moneyclaim.gov.uk or by completing form N1 http://s3-eu-west-1.amazonaws.com/hmctsformfinder/n1-eng.pdf and take it to your local County Court.
The Court will then issue a claim which a copy will be sent to the Defendant who will have a limited time to defend it, if not you can enter Judgment and enforce.
If the claim is for £10,000 or less it will be a small claim so you will not need legal representation. Over this value, you would need representation for trial.
Can I clarify anything for you about this today, please?
If not, I would appreciate a 5-star rating for my answer. If you need anything further I am available for a follow up at no extra cost.