Is this the one your after?
http://www.ebay.co.uk/itm/MG-ZT-TOW-...item1e7804db02
Ended without selling last week might be worth trying to contact the seller to see if its still available.
I was looking for a key for mine & contacted xpart this was there reply.
The keys only ever came as part of the original Towbar Kit. Our records show that the last supplier was B Dixon-Bate Ltd. phone 01244 288925.
I have called but number is no longer in use I also tried emailing but no reply which makes me think there no longer in business.
Not much help I know but I'd try the seller on eBay.
You might be lucky.