Some hotels already loaded in to the system in advance of the sale launch, so it is worth having a look around now. Note that the lowest rate showing on the results page doesn't include the flash sale rate, so you'll still need check the rates at each property.
As always, the lead-in rate is higher than the actual lowest rate. The Hilton Bradford is on from £37 and there are likely to be a few other hotels coming in at less than £50 as well.