In the UK 40 is at the top end of normal, 42 is where you get diagnosed with pre diabetes and over 47 is T2
Everyone no matter what their HbA1c would probably benefit from a little less biscuits, chocolates and cakes. For most people though a treat now and then does no real harm, the main problem is the amount of carbohydrates in today's diet.
Apart from the fiber which passes right through, all carbs when digested will cause an increase in blood sugar. Brown, white, wholemeal, gluten free will all have the same effect, the only difference is the white, bread/pasta/rice is easier for your body to process and will hit your blood slightly quicker than the wholemeal version's.
could all of the sugary food over Christmas be responsible for the rise and it’s just temporary
The HbA1c measures how much glucose as stuck to your blood cells over their lifetime, roughly 12 weeks.
So yes, any Christmas extras will have contributed. But if your glucose controls were working efficiently, that glucose would have been dealt with, the food isn't to blame, it your ability to deal with carbohydrates that has changed.
Do I need a complete diet overhaul or is cutting down on the chocolate, biscuits, cake etc enough?
As I see it you have got two choices, you can carry on enjoying your carbs until you are forced to do something about it because you are diagnosed with T2, to add to your other medical problems.
Or you can take action now, chances are you will be able to avoid adding to your medical problems, you may even improve some of the conditions you already have.
I know for me a lower carb diet improved my blood pressure, my IBS disappeared and my cholesterol levels came down.