Chris, it's the Sir Michael Beetham facility, around the back of the second hangar, and it's been there for years. It's where all the work on restoration, preservation, and exhibit maintenance is undertaken for exhibits from both Cosford and Hendon, since moving from St. Athan.
It's normally only open to Members on specific dates, normally in the evening, but I've just had the latest member's magazine (Radar), notifying of general open days, between Sunday 10th November, and Saturday 16th November, 10'15 to 13.00 hrs each day.
One of the photos in the Radar magazine covering the Wellington 'refurb' just shows what might be the fin and rudder of a Halifax in the background, looking very like the fin from the Hendon airframe recovered from Lake Hocklingen in the early 1970s.
If you haven't yet been to Elvington, near York, then it's worth going, just to see their Halifax recreation, using a Halifax fuselage with Hastings wings. Very impressive, and very big!
I posted a thread on the museum last year.