Manda, as Lyn says, if it's a Zometa (biophosphonate) injection, I wouldn't worry. My OH's oncologist sayid that there's now evidence that giving the drug at three-monthly intervals can work just as well as monthly.
Even If it's the hormone injection, there is still usually a few weeks time-lag before it really wears off. But keep nagging them, and try your GP. NHS funding is a mystery to me, but our GP's nurse sseemed happy to do the injection on request.
As for Christmas - it's hard, isn't it? An "experience" such as a ticket to a special event might hit the spot, if there's something suitable. Last Xmas, which we knew would probably be my husband's last (and it was), our daughters gave us National Trust membership, which gave us lots of pleasant days out for many months. I can't remember what I gave him - I think a bottle of good whiskey was in there somewhere! - but the main thing was to have the family around us and keep the day as normal and happy as possible, with good food, silly games, etc. Take some photos if you can.