I always worry going any later will mean she isn’t tired for bed. What would you say the latest I should let her wake up for her second nap should be so she is tired for her 7/7.30pm bedtime?
Some LO's do fine with a shorter A time to bed. It really depends on how ingrained the 7:30 BT is (some babies develop this body clock that has an unshakeable BT and they'll go to bed for the night almost regardless of how recently they woke), how long she has slept and how long her A times have been through the day. Bit of trial and error will help you get to know your LO's taste.
I tried keeping her up for longer this afternoon... so her awake time was 4 hours but she woke up again after 30 minutes crying. She went down easily though so I think she was tired.
That's a lot longer than 3:15... that's an OT nap - doesn't mean she can't cope with those 4hr, just possibly she's not used to it. Any luck resettling? If not, perhaps try again and just be ready to resettle.
How old is she now? Her sleep needs will reduce as time goes on and one nap will get shorter as she moves towards one nap (don't worry, this process take ~6 months.
Could it be linked to weaning? Perhaps I have cut back too much and she needs the comfort of having me around?
Yes, it could be this. Certainly its nice for us to feel they're ready to go to daycare and they'll be ok without us. However, when we're there and not offering, they can feel that we're withdrawing or something - DS did a similar thing when I tried don't offer, don't refuse to wean though he was a bit older. When she's at daycare, she will work out how things work there and they will be different from home, so its ok to still offer at home and know she'll be ok at daycare.