I know your car is a wee bit earlier, but on the 1999 +4 I used to own the clutch master cylinder was a Land Rover part. I had similar problems to you, but in the end I fitted a service kit, which replaced all the seals etc., rather than replacing the complete cylinder. It was cheaper, too.
I've since moved on to a Roadster and let the buyer of my +4 have all the servicing and maintenance records, along with all the receipts, so unfortunately I can't give you the part number of the repair kit, but I remember getting it from Britparts ( www.britpart.com
I seem to recall just giving them the serial number stamped on the cylinder and they could supply the parts to fit from that.
Maybe worth a try, anyway.