I'm not in your area but I was able to work out the figure for our area (Redbridge) quite easily.
The Secondary Admissions booklet supplied by our local authority lists all the schools in the borough and the number of places available at each one. With that information you just total the number of (state) school places available in the borough and work out the proportion of them that are grammar places.
In Redbridge it's 7.5%.
Edit: Sorry, I realise that it may not be so clear-cut if the grammar catchment area isn't identical to the local authority area.