Here I am again with another sleep problem

For the past couple of weeks we've had BT battles and it's taking an hour plus for her to go to sleep.
Usually in the past these kind of shenanigans have always been UT but I really don't see how that can be the case this time round?!
Routine for the past few months has been something like this:
Wu 5.30
Nap 12.30 - only ever sleeps 1.10 at childminders mon-thur
Bed 6/6.30
At weekends her nap would be around 11 for 2+ hours with bed about 5hrs later.
To try and get rid of the early start and add consistency it was recommended I keep her nap at weekends to 12.30 which I started last weekend and am going to continue to do so.
Now when I put her down at 6/6.30 she's messing about/then crying and this can go on and on. Even AP'ing isn't working! She's not really showing many tired signs at BT but at 15mo she has to be tired surely after only an hours nap??? I'm constantly doubting myself these days and so confused as to whether she's UT or OT..
I tried leaving her one night this wk to see if she might show tired signs a bit later but at 6.50 still none so I can't rely on them anymore it seems.
By the time she eventually does fall asleep she's incredibly OT and then often only sleeps 9/10hrs.
Any thoughts? How do I get her out of this mess?
I've lost count of the number of dinners that have been ruined where it's taking one of us so long to get her down!!