Sounds very odd but a black ant dry fly they can't resist......buggered if I know why though.
I used to make up some breadcrumb lookalikes and lob them near a large slice of floating bread for ok results.
Mullet go hard on fly.
Don't let your berley bread drift away, tether it near you.
Mullet spook very easily so be aware of your movements.
5lb tapered tippets are good.