I think it's 10%. I haven't bought from GBs for ages last time I did I didn't get discount - didn't have any proof
The last thing I bought was a Mavic wireless computer, I had to return it three times for a replacement, I asked for money back after second time they would only replace. When I took back number four I made a polite fuss and they finally took it back and replaced with a cateye wired computer which has since worked perfectly.
In addition to the 10% discount you normally get served in an offhand manner too, I think they give that to all customers mind, not just ACC
Doubtless others have had different experiences but for me personally, I'd only buy ACC kit from there (cos I can't get it anywhere else) - and through gritted teeth. Nice bikes, shame about the attitude.
Anne Snoopwatch Robinson