Hi Bob,
You should be able to get replacement chokes + key via the UK importer/distributor who if I remember rightly is GMK of Fareham Hants, thats if your local shop cannot help.
I had this problem with a semi auto and Edgar Brothers of Macclesfield who distribute my particular gun kindly supplied me with what I needed free of charge!, they were only to happy to help and were very friendly. All it took was an email to them with the serial number of the gun.
www.gmk.co.uk or give them a buzz on 01489 579 999
I cant say that they'd do the same for you as what Edgar Bros did for me, but you'll never know unless you give it a try
I hope that helps B)