I got rooms in the Hilton on lastminute.dom recently for £63. The best deal seemed to be on a night by night basis, only snag was you get them at the lastminute ie the day of the reservation. There were other offers on lastminute as well though.
BTW the Hilton at London Paddington station is a great spot. Its the old railway hotel and its directly linked to the station, so if the kids are into trains and all that then you are bang on. As well as this there are a number of shops, pubs and restaurants in the station building so its all there.