Advice please - winter wedding, evening dress

[SebbysMum]

Going to my cousin's winter wedding in three weeks time. The dress code is black tie and she has made it known that she would like all the women to wear long dresses. I have one, nothing special but it will do as I can't justify buying a new one just for this event. It is long, deep purple with straps. I haven't got a photo but this is the closest picture I can find online, from the front only.
example dress

How would you accessorise a long dress for a cold winter wedding? Complicated by the fact my grandmother will scrutinise whatever I wear and probably expects me to wear something on my head. Surely evening dresses and hats do not go together?

fur shrug, no idea on shoes, um could you get away with a fascinator instead of a hat?

evening gloves!

i'd be looking at grey/pewter to go with purple e.g. shrug, gloves. pewter's big atm, should be easy to find shoes.

if i could walk in them i'd wear these shoes (closed toe for tights) but may be a bit trashy for your taste.