The bathroom company (who are generally excellent) have said it's very very difficult to take up tiles, add more heating loops, then put it back down. They said the heating is a factory sealed unit, and while it might possible it's difficult, and it voids any warranty.
I've come up with an alternate plan. We take up a few tiles (3-5) around the toilet, put a metal sheet over the heating mat and over the unheated areas, then put new tiles down. That transfers the heat sideways, then lets it go up through the tiles. This would result in the same amount of heat being spread over a larger area, so it won't be as warm, but it will be a lot better than dead cold.
Does this sound like a good solution? Can anyone see any downsides I can't spot? Should I insist on them adding more heating element instead of this workaround?