Definately give Sean at Eagle Flight a call, he's pretty flexible on times. I did most of the flying for the night rating with him last Christmas and it included flights well into the evening (landing at 9 or 10pm). The only thing I couldn't do was the 5 solo take-offs and full stops as you can't fly solo from Luton without a licence (I was doing the night to make up the hours towards the PPL)