Another update that might be helpful (not sure if you're doing the DF always at the same time, if so then this probably doesn't apply to you).
I was doing the DF at around 10pm and DS was waking up at around 4 hours later - two nights ago 4.5hrs later - which always happened to be at around 2.15am. If I didn't do the DF he would wake up at around midnight.
Last night I done the DF at 11pm and though DS was going to wake up at around 3am - no chance, he woke up at 2.15am. So instead of feeding him I done the pat/shush and he went back to sleep until 3.20am, when I done it again and he slept until 4am - when he woke up REALLY hungry and I fed him.
Will try again tonight, but it was obvious to me that even though he would have usually taken a full feed at that time, he didn't really need it, just got used to having it.