Carrycot to fit old style mountain buggy terrain?

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In preparation for our newest arrival, my friend's kindly given us her old Mountain Buggy double Terrain (roughly 8 years old) but unfortunately it doesn't have the carrycot to go with it. I've seen quite a few carrycots designed for the Urban model on ebay but I don't whether they'll fit our one, and as our model pre-dates the Phil and Ted's takeover, Mountain Buggy are unable to advise us on which carrycots will fit our Terrain.

So, in summary, does anyone know if you can fit a carrycot made for a double urban to a very old double terrain before I go splashing the cash on ebay?