I have come back from a great week in Morocco with about 1000 dirhams (which you're not supposed to do). Does anyone know how I could change them back into pounds sterling?

Try taking them to a bank that offers currency exchange services. If that doesn't work, perhaps you can "sell" them to someone going to Morocco.

I just got back, and from what I understand, dirhams aren't exchangeable internationally. So I guess the best bet is the previous suggestion to find someone who's planning to go.