Today in quite gusty conditions I found that as I did the big bear away, the board kept on spinning out on me. I was trying to keep as little backfoot pressure as possible to counter this but it was a little hard in the huge gusts.

Any hints as to how I can overcome this?

I was running a 5.0 Koncept on a 85 Sonic with a 24cm Lessacher weed fin. Mast base was a little forward of center of the track and boom was about chin height.

Thats a pretty small fin in that board how were the water condition choppy or flat.
I had a Isonic 86 smallest fin i used in that was 28.

It was a little choppy.

Red uses the same fin (24 Lessacher Chameleon) and has used it on this board and I dont think he had an issue with it.

Some of the Lessachers I've seen are a bit thick up front, that can cause ventilation as you pick up speed, (around the the 30skt mark).

How are you harness lines? if they are forward of centre of effort, that will lead to too much backhand and back foot pressure.

I am pretty sure the harness lines were pretty well balanced.
The more I think about it the more I feel it was bad technique. Due to the gusty conditions I think that as I veered downwind that maybe I moved more towards the back of the board just trying to stop from being thrown forward. Maybe I was putting too much pressure on the back foot.
Anyone have tips for bearing away in overpowered gusty conditions?

in overpowered conditions move your lines back a tad and that seems to help also instead of sheeting in with your back hand to bear away try pushing away with your front - puts less pressure on the fin...

Works for me..