In uk.d-i-y Gary Woods wrote:
My dad, who started his career as an aircraft mechanic called that "Brute
force and foul language." I've seen similar fastenings, and they generally
were not meant to be taken apart. If you can get at the ends of the
prongs and lift one at a time with something like a flat-blade screwdriver,
you might get them to release*, but I'll bed they'll break easily, leaving
you to, perhaps pin the works together with a drilled hole and screw.

Hozelock do sell spare winding handles, though you'd have to check they are
available for your model.

Theo