We booked our Hilton for May and September back in the January sale, as it was a complete bargain! Thankfully we fly Virgin, so go from the South terminal. However, it's still very easy to get from the hotel to the terminal. We've done it two years in a row, with 3 people, a wheelchair, 3 suitcases and 3 sets of hand luggage! It's simply down a corridor, into the car park, and in through a door to the terminal!
Normally, it's a quick hop on the monorail, but obviously it will be a case of getting the replacement buses to get to north terminal. But I would imagine they would be pretty close to the main entrances and well sign posted! Well, one would hope...