From the Save The Train Forum :
Located in land between the triagle of tracks enclosing Didcot Station and the GWR Centre, work has now started on this new building. Foundations are currently being layed, but upon completion the building will be 3 storeys and 75m by 21m.
It will take over current signalling for the Reading area when the station re-development is complete, along with Swindon 'B' (which controls the Didcot area), and I believe Slough and Oxford signalling may also be transferred there?