In article ,
says...
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The one I have is like this
http://www.plumbworld.co.uk/1630-13292
which has holes which can only be screwed into horizontally.

Common sense would suggest a block of wood screwed to the floor then
screw into that, but for the WC pan the u-bend is in the way, so no
joy.

I thought that was what you meant, but I wasn't sure.

Fix screw eyes into the floor, ram a bit of dowel through the eyes and
screw into that. If there isn't room for that you'll need a couple of
steel angle brackets with some sort of captive nut arrangement. In
either case silicone would be a worthwhile addition.