Ok, Ironman diversion over, back to baby quilt making. I have managed to sew all the sides to the white fabric centre. There were some serious sweating over cutting out the centre to the right size but it turned out fine with a little jiggery pokery. Probably this would be vastly helped by having a longer quilting ruler and a larger cutting mat, but I am sure there is a technique I have yet to stumble across using only stumpy starter tools.
Unless you are very confident in your cutting I would recommend pinning the side to the white fabric , turning it back to check it meets where it should and then putting four or five stitches to secure it in place before continuing to sew the rest of the length.
This avoids any unpicking (ahem, as if).