You won't automatically get maintained at Diamond. If you were very close, and with some stays booked early new year Customer Services might be do something for you but I wouldn't count on it.

How much do you value the diamond benefit? Whether its worth chasing is only something you can decide. If you visit lots of places with proper exec lounges, then the benefit of access irrespective of upgrade availability might be worth it. Potential for better category of upgraded rooms?

5k base points would be around £330 spent on one or two nights in the UK - with food charged back to the room that's easily achievable, especially in London. Remember you need to check out on or before 31 December.