Second attempt ... much better!
Blogland is such a great resource. No sooner had I posted about my test block being slightly smaller than it was supposed to be than I received lots of encouragement and some very helpful tips on how to get perfect seam allowances. I spent some time this morning working out where to sew to get the right seam allowance, marked up the bed of my sewing machine with some electrical tape (thanks Andy!) and sewed up a quick test using off-cuts before having another go at the basket weave block from The Farmer's Wife sampler quilt.
This is a much better size (a smidge less than perfect but well within my tolerances!!) - a good half inch bigger all round than the last block. Sadly I placed the fabrics in a different order to last time and didn't notice till it was too late, but in the scheme of things I don't think it's a major problem and I draw the line at trying this block for a third time!!
Off to plan my next block and probably to douse my ironing board with even more spray starch! Hooked!