There is math for all of this MrsEggy, I'm just not the one to provide it.
The reason I asked about extract is because sometimes extract just sort of stops fermenting around 1.018-1.020 and there isn't much a person can do about that.
Safe thing to do is let the beer keep fermenting another week and check gravity again. Three - four weeks with high gravity beer ( above 1.060) is not uncommon and won't hurt your beer at all, it usually helps.
The yeast we all love so much are on their own course of time. Generally they like time, quite a lot of it actually, which is nothing a person would gather reading most recipes from stores.
Time is on you side now, and time will be on your side when you carbonate the beer. A few months from brew day to popping the top off a well carbed bottle, especially on a higher gravity beer, is perfectly normal.
Check your brew in another week, if the gravity hasn't dropped. Prime it with some corn sugar, Bottle it and let sit a month or two at 70-75 degrees.
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