Remember that continental Europe is 1hr ahead of the UK, so a flight leaving Heathrow at 9pm UK time is actually 10pm on the continent....
As before, easiest way is to look at the departures boards online.
Also. what's the driver to get a very late departure? If you're looking to minimise cost and maximise time in London, there may be some work arounds - a late train to Birmingham (£10pp), a night in the airport Novotel (£50) and an early flight on from there???
DY - is that Norwegian? - have flights to Bergen at 8.35pm and Stavanger at 9.05pm tonight from Gatwick..