Once all the debris was retrieved and I was packing stuff away, when I put my FF9 transmitter, which has a plug-in Spektrum 2.4 module, back in the box, I noticed that the brass threaded stud on the back where the aerial wire is attached, was loose, the brass securing nut had come undone.
Hence the mystery of why I lost control and the model going in, it went into "Fail safe"
My local LMS told me that it was a known (not to me) problem, as when it comes loose you get interference, hence the lock out.
The cure is to Loctite the nut, which I have now done.
An expensive lesson for me to find out about, so if you have one of these units, check yours out, and Loctite it anyway, so as to avoid my pitfall.