this exact thing happened to my sister a few years back.

Just because the guy behind didnt SEE you indicate, Dont mean you were
not!! (and you know you were)

Stick to your guns mate, the guy was in the wrong and you all know it!!

hope you get it sorted.

as long as you insist you looked in your mirrors, and were indicating
(and make it know the witness is a mate of the bikers! lol)
then you should be ok.