Delving deeper in the bag of blanket wool I espied a full ball of a beautiful lilac double knitting.  I was puzzled as to why it was there but Aunty Biscuit explained that not all DK is the same thickness and that generally some do 28 rows to 10 cm and others 30.  She had been given the ball for blankets as it is quite thin.  Seemingly ball bands have a little graph on the side which will show the number of stitches and rows which will produce a square when using the needle size they show.  Apparently this can be handy if you are wanting to use a different yarn to a pattern although you would need to check the length in the ball as it may mean you need an extra one.  It all sounds very complicated to me.