Went for a ride today and definitely not serious enough to warrant a new bike. Think when I filmed that I was sat a bit upright as my arms aren't normally near locking.
I'll try the longer stem and see how that goes. I've done nearly 1300 miles on it now so can't be all bad, but I do agree that with hindsight I should have gone one size up in the frame.
Can't decide on saddle height, lowered it slightly today after seeing the hip movement but think it may have been right where it was!
I'll probably keep scanning the suggestions in this post and tinkering based on them for a few more rides to see what does/doesn't help.
Cheers guys :-)