sto635 - SAR deregestration support

Rob,

Please can you review the homestead changes for sto635? No rush on this obviously, as we don't need it until the rest of the story is complete.

I have written complete UTs for this too, but they're on the handlers_ut branch (with the rest of the handlers UTs) and there isn't a great deal of point in you looking at them as none of those UTs have been reviewed and this stuff is heavily integrated into tests I'd already written.

Cheers,
Graeme