Hi
Change your flights from Belfast to Heathrow, I honestly would worry about this connection. I sat on the M25 three Saturdays ago for over two hours and it wasn't even rush hour.
Did you book this journey with one through ticket directly from the airline? If you did then contact them about your problem and they will hopefully solve it for you.
Booked on two tickets, say Easyjet to Gatwick and then on with maybe Qantas or BA from Heathrow, the forget about the first flight and rebook yourself on an earlier flight. Note this is only a solution if there are two separate bookings.
Booked through an online travel agent, then it's time to get on your knees and pray to St Jude.
Jackie