Infrequently, as with all the Inter City outfits.
Firstly, they are innundated with CV's from already qualified applicants.
They also have a comparatively "young" driving workforce. At privatisation, the more senior hands ay depots like Euston, KX, etc. went to the inter city operators. Most of these retired within a few years, so there was a wave of mass recruitment by them a few years back, and yes they trained plenty from both internal and "off the street". That process is pretty much complete, so openings are few and far between, as there aren't going to be many retirements.
The way into these companies is as a qualified driver from elsewhere. As for trainees, think single figures per year across the route, if any at all.