After a chat with the technical support person at Paterson (who was mildly surprised that Aculux 3 does not detail Plus-X times), I found out that the times should be the same as D76 or ID11 as 'the Aculux times are based on them'. (This confused me a little as the times for Plus-X in both are 5.5 and 8 respectively.) Nonetheless he was surprised that a 6.5 min time was producing thin negs and suggested out of date developer. I ran again tonight with an increased time of 8m, and again got thin negs with even the neg leader (that I thoughtfully included) being semi-transparent. The long and short of it is that the 'new' bottle of dev looks to be out of date and I will call the supplier and get a replacement before attempting Aculux 3 again.