First, check if there is anything happening at 11.30 that might disturb her. Does the temperature drop because the heating as turned off? Are you coming to bed and waking her? Is the white noise automatically switching off? Does the boiler make noise? Check everything in her environment is the same all night as much as possible.
Second, set a wake up time. Even if she’s had a terrible night, you get her up at the same time every day. I would say 7am, but if she wakes any time after 6, you start the day.
How does she fall asleep? Try to have her fall asleep in her cot, without a bottle. You don’t want anything to be different when she rouses overnight as it can be confusing and distressing.
Be really consistent with her night time wakes. No milk. Be reassuring but boring. Try to avoid lifting her out of the cot if you can, and just repeat the same phrase eg. ‘It’s sleep time’. Bum patting and shushing can be helpful.
I would probably try to reduce her naps a little bit but gradually shortening the afternoon nap 5-10 minutes per day.
Lots of fresh air and afternoon daylight every day will promote melatonin production and reset circadian rhythms.
But also remember, night waking is NORMAL. It’s shit, and modern lives are just not set up to deal with it, but it is biologically totally within the realms of typical development. In tribal cultures, you would have extended family to nurture you and bab. You’d go to bed earlier and co-sleep, and have time to nap. You’d share wet nursing with other women. Our bodies and our babies don’t know that we are now expected to work, and live as isolated family units, and run a home. So please, please cut yourself some slack for finding this hard.