Loose Toilet

I have a toilet set in cement on a tile floor that rocks when you sit on it. How do you fix it? Is it actually bolted to the concrete? There are two holes in the side of the base but they are full of cement.

Neville, the pan should be screwed to the floor through the 2 holes you mention either side of the base.
The best solution is to clean out the cement from the holes, mark the position of the holes on the surface under the pan with a narrow texta or long nail etc, remove the pan and drill into the cement to fit plastic plugs to take new screws.
If the cement base is not firm you may have to clean it back and start from fresh.The cement may have been originally laid to take up any unevenness in the floor but if it looks ok you can clean away all the cement and use a bed of silicone in its place.
If there is a timber floor below, screws (brass) long enough to fix directly into it will do the job.
It's a straight forward procedure. Just take it one step at a time & watch out for the water when you remove the pan !

Thanks Peter. I will give it a go on the weekend.

My toilet is sitting on a concrete base about 10-15mm thick which then sits on top of the floor tiles. I plan to do a bathroom renovation soon, including replacing those tiles, and I have already purchased a new toilet suite. My question is this... I have sat the new pan on the tiles and there is minimal movement, about a 3mm gap between the front of the pan base and the floor, so do I need to re-lay the concrete base or can the new pan be installed on a bed made of silastic etc.?

Its common practise these days to silicion and screw the pan to the floor rather than set it. It removes the need of cutting tiles around the pan - you simply tile and sit the pan on top.