Evening all.
OK, had a quick look tonight and it was the magazine indexing screw. Just followed the instructions in the Steyr service video, and job done and I'm a little bit wiser now with the H5. Shame there aren't more service videos on the H5 but hopefully I won't need anymore anyway!

I have to say part of the reason I have three Steyrs is the ease of maintenance, partly due to the service videos.

Thanks for the steer Tinbum and Rich (should have gone straight therein the first place).