I seem to be a bit stuck on what I thought would be quite straightforward.
I've created a fishnet of polygons, all 25 x 25 metres, 100 polygons by 100 polygons.
My main task is to create a relative co-ordinate system based on the distance from a vertex somewhere near the middle of the grid (but NOT the middle), which will be the origin 0,0.
Basically, the polygon square - where the origin sits at the very bottom left of it - will be X=0 and Y=0. The next square to its right will be X=25 Y=0 and so on and so on to the extents in all directions. The increasing blocks of 25 just represent the actual distance in metres from the origin.
I thought I could maybe number the columns and rows of the polygon grid then work from there, assigning an X and Y value based on column and row number but I can't even seem to do that.