Thanks!
I found 7 frames gives about the right speed for the transition (of course it’s relative to the video mode - so it would be too fast for a 50fps video). I thought of explaining that but felt it would complicate things…
Thank you! LOL! You don’t want to be British though- our weather is rubbish… 
I’ve decided I deserve something stronger than just tea …
Thank you so much - I’m gad you picked up some tips!