I've got all of the way points for "Centre of the Circle"

C'mon. Even your 450 will be able to do that (I think). No need for Mapsource.

To find the center of a circle going through three known points, all you need to do is:

1- find the point in the middle of A and B and the bearing from A to B

2- find the point in the middle of B and C and the bearing from B to C

3- project a waypoint from midpoint AB, +/-90 degrees from the A-B bearing (depending on the side you need to project)

4- project a waypoint from midpoint BC, +/-90 degrees from the B-C bearing (depending on the side you need to project)

5- create a direct route between AB, BC and their projections

The center is where the two lines cross.

Otherwise, you can also use sinus and cosine, but it's takes more time.

EDIT: Added a visual representation (made on... Mapsource). But I tell you, all of this can easily and quickly be done on your GPSr in the field.