Welcome frankie and first mum!
Here's a link to the basic info about the transition if you haven't already seen it:
From 2 to 1 nap transition (10-12m and older)Ladymugg and I were in a similar situation until my DS started NW and EW. It may be due to a few things... teething, physical and language development, emotional dev't= separation anxiety, etc...
Our routine at the moment is still forming, but I will say that many times during the transition, we did a routine listed below because DS just could not handle more than 4.5/5 hours until recently actually. It did have to do with poor nights and EWs, so take that into consideration. Also, my DS often refused late pm naps, CN included so we did them in the morning. Well, at the very early stages of the transition, we did do very late CNs around 4 or 5 and bed 2-3 hours later, but once BT gets pushed so late, you have to just go for the 1 nap.
if...
WU 5 o'clock hour or earlier =X
CN 9:00/9:30, 15-20 mins. in the car
N 11:30 (or 2 hours later on the dot)
BT 6:00
if...
WU 6:00
N started transition at 10:30, pushed to 12:00 slowly (15-20 min a day, finished in 10 days or so)
BT 6:00 at the beginning, 6:30 towards the end for a good 1-2 months
For only a couple days now we've been on this routine, but it does include many NWs and early morning resettles.
WU 6:15-6:30
N 11:30/12:00 (usually 1hr30min = 1.30)
BT 7:00-7:30 (because DS seems to sleep avg 10.5 hours lately)
DS does seem OT, but I honestly don't know what else I can do without getting a 6:30-4:30/5 schedule or the likes. He seems to enjoy his evening lately so I'll be on this for a few more days. It does take 5-7 days for new routines to stick so I'm hoping our 5am WU and resettle will go away.
Hope there's been progress on your ends... hugs and best wishes.