Thanks for the replies - yes, I did post on both Kent and Sussex as Groombridge is so close to the county border.
I had taken a little while to find the Cutler-Ilsley marriage, as I initially looked just before the birth you identified in 1920. In fact there were only 2 children, the second in 1923, also in Birmingham. They were in Floyer Road, Small Heath, Birmingham at the time of the births, and are together on the voters roll there in 1925.
However, it had seemed strange to me that there was the 8 year gap between marriage and child, it may that the war was one reason (I have yet to investigate his possible army records). I wonder if they both went back to Groombridge after the marriage.
One final thing, John is said to have worked as a Royal chauffeur at some point, there is said to be a photo somewhere of him driving the King (George V) but I have yet to look into this!
Thanks again.