Thameslink use Cauldwell Depot (or at least sidings there) for 700 stabling but not the servicing facilities. There are a number of workings into and out of the depot each day
Train information at Bedford Cauldwell Depot between 0200 and 0159 on 04/02/2021. From Realtime Trains, an independent source of train running info for Great Britain.
www.realtimetrains.co.uk
A look at a satellite picture shows why the depot isn't any use for fixed formation 8- or 12-car units, the buildings being suited for 4-car units.
I think there was some sort of reorganisation to allow 12-car units to be stabled there as there is an awkward disconnect between the 14 8-car roads in Bedford Carriage Sidings and the 12-car operation of Bedford to Gatwick / Brighton services.