I've finally tracked this down to a misaligned 16-bit access in the template function for...
It seems it is necessary to add (QT_ARCH_AVR32) into the list of arches which can't handle 16-bit non-aligned access.
I guess I was expecting this to be blindingly obvious once I got to the correct place in the code.
I've not looked at what the compiler generates here, but I believe ARV32 can do some non-aligned operations - seems not in this case.
Thanks for help on this - got there in the end & can now move on and do some applications work.
I'll post this in other places.