We have a block reservation at The Coseners House in Abingdon (http://www.stfc.ac.uk/About%20STFC/44.aspx)
This is probably the nicest option nearby.
These rooms are held until 10th September - at which point I have to release them - or pay for them. If you contact Cosener's House to book say it is for the Quattor Workshop.
The rate is £77 per night (£57 with shared bathroom).
Alternatively, there is Ridgeway House at RAL itself (http://www.stfc.ac.uk/About%20STFC/54.aspx).
There are also various hotels in Didcot.