I've stayed at The Euro - near Russel Square and enjoyed it. It's in an old town house facing Cartwright Gardens. Very modestly priced. Has everything from rooms with shared bathrooms to nice rooms, ensuite.
Euro Hotel
Location is great if you are coming in from Heathrow - since it's 2 blocks from the Russel Square tube station - which gives you a direct shot to Heathrow. It's also walkable to the circle line...
And very close to the British Museum!
We have always been successful in bidding for London hotels on Priceline.
We like Kensington. Tube straight from Heathrow or Easyjet bus to Gatwick. Have had Marriott Kensington and Marriott Maide Vale a few times.
You might try the Premier chain.