Hi Nicole, thanks for replying. I think I may have found an answer, albeit not a perfect one....
She rolled onto her tummy yesterday while I was trying to put her down for her 2nd nap and fell asleep pretty much straight away - slept for 2 hours! Tried it again for her 5 o'clock nap and she slept for 40 mins (about right I think for her routine - 4 hourly feeds and bed at 7pm).
So, today - she woke at 6, I fed her at 7 - she had had an unsettled night as I think she has a cold coming so was yawning and rubbing her eyes by 8. Put her down on her tummy and she has now been asleep for 1 hour 50 mins!!!
I know it's not ideal, and I won't do this at night as I am too nervous, but it looks like she is far more settled this way......
Here's hoping she doesn't have too much sleep in the day now!
Cheers.
(a much happier) Lisa