Thanks for your help nicktheplumber
the toilet itself actually sits on a mortar bed about 3/4 inch thick on top a concrete slab floor (linoleum covered)
the mortar over time has cracked and caused the toilet to become unstable – so now needs to be replaced.
I’m assuming that this is a different method of installation to other countries, as the mortar is the only thing securing the toilet in place. – This seems to be the norm here in Australia