Took your advice and visited DubJam for lunch on Monday. It was really good; nice atmosphere and friendly service, but not the cheapest (then again, I have to think in central London terms - it wasn't too bad!). The only criticism is that the menu is a tad restrictive and by the owners (??) own admission, it has been severely westernised. No curried goat or saltfish and ackee (the national dish of Jamaica) for instance.
I went to the Jamaica Patty Company a couple of times (just round the corner in New Row), where the patties were the best I have managed to find in the UK..
Thanks for the heads up.