Terrible terrible bedtimes with nearly 2 year old - please help!

[Girlsville]

Hi everyone
After some advice with my 23 month old DD1. She has never been the best at going to bed - as a baby used to cry a lot but we had developed a happy solution of sitting next to her bed patting her back for ten minutes and then leaving and she self settled nicely and sleeps through. By teh time she was about 1 she didnt need patting any more.
However, we are going through an awful phase of terrible bedtimes which ahs been on and off for last three months. Every time it gets better for a few nighst then awful for a few nights.
Current routine is upstairs for bath at around 6-6.15 with her 8 month old baby sister. Both love the bath. DD1 then sits on the floor, sips her milk, and reads books while I feed DD2. DD1 then "helps" me read a book to DD2 and we put DD2 into bed around 630/645 depending on how tired she is. Luckily DD2 then falls asleep without a peep.
DD1 and I go into her room, and we then have about 15 to 20 min of Mummy/DD1 time, reading stories cuddles etc before I put her into her cot around 7pmish. She is fine going into her cot and either lies quietly or chats away but then screams blue murder when I leave the room. Over the last three months have tried (each for several weeks at atime) variations on returning every 1, 2 3 misn to calm her down, or staying by her cot for 20 mins before leaving etc. What normally happens is she cries on an doff for anything between 1 - 15 mins then chats for about an hour in dark to her teddies then whimpers and falls asleep around 830 - too late for her as she is then tired the next day. (she has a good nap from 1pm ish to 3 -330ish too).

I wouldnt mind staying with ehr until she falls asleep but because she talks so much this would be for 90mins which I am not prepared to do. I think part of problem is because she naps until after 3 she isnt quite ready for bed at 7pm, but I cant really make bath bed etc much later because my 8 month old is so tired and needs bathto revive her at 6ish. Today have tried to make DD1's nap a bit earlier at 1230 in hope she will wake before 3 and be mroe ready for bed tonight.
I cant bear the screaming when I leave the room, but cant work otu whats going on as she is happy chatting to teddies after a bit. All I want is a nice calm bedtime.
ANY THOUGHTS? PLLLLLLLLEEEEEEEEASSSE!

I'd suggest dropping the nap. Perhaps she just isn't tired enough at 7. I had a similar problem when ds2 21 months. Made him stay up all afternoon and it helped. Couldn't go far in pram or anywhere in car for a few months as he'd drop off!

Thanks weasle
Today I changed nap - she slept from 1230-2ish so I knew she was tired at 7pm. She was fine for 20min while I was in room, when I left she screamed blue murder for 5 min then went straight to sleep. Youa re def right taht when sh esleeps until after 3 she isnt tired enough, and I think the screaming when I leave has just become a nasty habit which i have no idea how to break!

My DD is roughly the same age and also still naps for about 2 hours, I wake her at that point if she's still sleeping. The absolute latest I can let her nap until is 2.30pm if I want her to sleep about 7pm (and I try and let her run herself ragged in the afternoon). Hopefully if your DD is more tired by bedtime the screaming will soon stop.