If you are really stuck getting to Heathrow, I live near Heathrow on
the 285 bus route, which goes to T3 and is a 24 hour bus service.
If you find no other solution you are welcome to stay in spare room
overnight and get the 285 bus in the morning if it helps you out.
It's about 20-25 minutes on the bus from my location to T3.
Send me a PM if you want to take up the offer.

Of the other suggestions made so far I think a minicab to Ealing and
then train from there sounds quite good.

John