Thanks for all your replies. Unfortunately resettling is not an option for 2 reasons - I have a toddler already who wont wait for anything so I dont have the time to do it and then the said baby wakes up reasonably happy so it would be so hard to get back to sleep. Its fair to say that Ive gone back to 3 sleeps for the moment by using the pusher and rocking him that way to get him to sleep at 4pm for about 30-40 minutes and putting him to bed later (after 7pm). This, so far, has resulted in his night feed to move from 4:13am to 5am so far, but still wakes up at around 7am. His happy awake times are starting to stretch out to 3 hours in the morning and about the same for each other bracket. AndiE, Ive had probably the most similar situation as yours where I've struggled to get into a 'set' routine, which is what is affecting this whole game.
I might wait just a few more weeks and then see if he gets a bit stronger to last until 6pm from 2pm as Im preferring the 5am wake up over the 4:13am.
Decisions decisions! I've thought about bringing that last sleep of the day forward a bit to see if the baby gets that he has an option of stringing the 2 sleeps together in the afternoon, but haven't tried it as yet.....and not overly sure it would work.