I have just been browsing through some prices for a trip to Heathrow at the start of March, anytime between Thursday 3rd and Sunday 6th. Flights are around £50 return (with BMI to LHR and EI back again), and a hotel for the night is £29 for a twin room at a Premier Inn right next to Heathrows RWY 27R.
If anyone is unable to go to Madrid which is later in the month or fancied a trip to Heathrow...