Over Christmas we flew AMS - LHR T5 - JNB, a freak storm in AMS meant we arrived into LHR T5 50 min before departure and they still wouldn't allow us on the flight. We were put up in the Sofitel at T5, assigned 2 rooms (we could've asked for 3) given an overnight bag and food vouchers and rebooked on the first Virgin flight to JNB. I was saying all that to say I don't think you would make it especially as you were travelling between T5 & T3, but I see that your flight has been further delayed anyway. Good luck for tomorrow, I hope you at least get to LHR tonight!