Thanks so much both of you for your advice.
Nicole, I got the same info from Michelle-megsmom about earlier bedtimes and it has absolutely sorted the bedtime screaming. Heres what has happened the past few days since writing in: Erin has been waking about 6/6.20am so couldn't make it all the way to lunchtime for one nap and has infact been tired and ready for a nap about 3 hours later. I'm letting her sleep no longer than an hour so we can still fit an afternoon nap in. So, she is awake from nap one by 10/10.15 and starts to get tired again at 2 ish. Again, I wake her after an hour as from experience I find 2 hours napping a day seems to suit her. She is awake then by 3. What I was doing was putting her to bed at 7 ish but I think it was a bit too long for her and I totally agree with you that as little as 15 or 20 minutes can make all the difference for a happy little girl at bedtime or complete meltdown!! She was obviously screaming and crying herself to sleep and so when she woke in the night she just got upset again. Instead, I have been getting her upstairs and having a longer wind-down time right near the bath and bedrooms so that I can catch her at the right moment and get her to bed sleepy but awake. I haven't had anymore upset. It's been great. She hasn't woken up in the night either.
So, there was just the waking at 4/5am to deal with. That has still been happening and I am only getting her to sleep til 6ish by picking her up. I could not think what on earth had started to wake her at that time until yesterday, I went in and there was the loudest scratching and tweeting noise above her in the attic. A bird has only gone and made a nest in the boards above her ceiling and it is so loud!! I thought I heard something a few days ago but that was just the mother bird then. Now, the eggs have hatched and at first light the mother is feeding them all, which is a noisy business I can tell you. Poor Erin must be in a light sleep and it's waking her and frightening her. We are just going to have to ride this one through until the birds fly away and we can block the hole up!!
What a nightmare! But thank goodness I have found out what's been causing it!
Thanks so much for helping me out though. I am going to keep to two naps for the time being until she is either waking in the morning a little later or can handle more awake time. May try again in a few weeks x