The basics of a christmas dinner is already pretty sw friendly. I have a roast quite often and as long as I do the potatoes with frylight it's one of my fav slimming world dishes.

What other trimmings do you have with your christmas dinner? is it the trimmings that load it with syns?

It can be. Gravy has Syns, pigs in blankets are covered in Syns if done properly lol (my downfall!!)
Plenty of people add butter etc to mashed carrots, mash potato, etc. So there's always Syns there too.
And Yorkies... Can't forget the Yorkies!

The new SW Christmas book (well not completely new as I got it last year but it's back out for this year now) has plenty of ideas for a syn free Christmas meal.
Like you said the majority is already free, if done with out the extras (butter, fat, etc.) so it's just finding alternatives to those to make it free or less synned