Thanks Chilli and Grabbers
As is often the case, the answers received here are a good description of what should happen, but not of what has actually happened.
The Notam simply states that the airfield is shut.
"25 DEC AD CLOSED. UNAVAILABLE TO ALL TRAFFIC."
No mention of CTA or CTR hours.
I have checked the Enroute Notams but can't find anything.
The airfield plates on the IAIP direct you to ENR 2.1 (Enroute) which tells me the hours are as advertised and as advised by Notam.
....... So I rang the airfield in question. They were pretty clear that the CTR would revert to Class D but less certain about the CTA. The CTA is shared by another adjacent airfield (which is also Notamed shut on the 25th so it's a fair bet but....)
My understanding is that the SAM STAR and SID for Gatwick and Heathrow all route via the Class A above the CTR/CTA so should not be an issue.
I'll give London Information a call once I'm airborne (weather permitting) - it's a good bet that they won't know either.
Thanks again for your help guys.