It's not a good idea to assume the Tegula circle will be exactly 2500mm dia, John. If you get all the joints at 4mm instead of 3mm, you can be 12mm out by the time you get to the outer ring.
This is how I'd do it....
Establish centre, then set kerbs on semi-dry mix at 1.3m radius (2600mm dia) and place haunching concrete to back, but don't compact solid.
Screed bedding layer for circle, allowing for fall from centre to edges (25mm is about right), then lay the circle starting with the centre stone monstrosity and work your way out. When you get to last ring of Tegs, bed them on extra haunching concrete and push up the kerbs to suit, so that the backs of the kerbs are tight against the edge of the blocks, which should be approx 3mm higher.
Use a rubber mallet to knock down the outer ring blocks to the correct level, which will compact the haunching concrete at the same time, and then, when everything's in place, haunch the front of the kerbs to prevent them being kicked out, or lay more blocks.
Leave it for a couple of days before wackering, to give the concrete a chance to set.
HTH :)