Is this a simple question of wake time that I need to increase...
I would go the opposite way and encourage more sleep, not less. If baby isn't sleeping for as long, then I would make my DCs wake time less, so that naps are more frequent - more naps per day.
In my experience it is time asleep that drives the change to longer naps, not time awake.
was a catnapper for ages but then at about 5.5 months started taking proper naps
Short, frequent naps are "proper naps" for a younger baby. 40-80 mins awake followed by 20-40 minutes naps repeated throughout the day is just how a young baby sleeps. The change to longer, less frequent naps happens somewhere around 5 months - 9 months old.
It sounds like your DS is transitioning to the stage of longer, less frequent naps. But that he's not quite there yet. The process of transitioning isn't an overnight change, it is gradual. Not unusual that you will have some days with fewer, longer naps and other days with more, shorter naps - until the new sleeping pattern becomes fully established.
So a child napping for longer will naturally have longer awake. But a child having shorter naps is likely to have shorter naps regardless of awake time. So increasing awake time when nap times are short just creates over tiredness problems.
I'd cut awake time to tackle the build up of over-tiredness, so have lots of frequent, short, naps. Then once baby is no longer over tired he may well extend his naps naturally, which will naturally lead to longer time awake and so fewer naps per day.