Argh! I had a look through the yarn I'd bought and 3 of the balls had no label on so it seems they must have been from a different dyelot. Unfortuntely the yarn all looks the same colour in the ball so it's not until I was actually knitting it that I saw the problem.
I tried frogging back and then alternating two balls of yarn but it's just turned out stripy :(
I've been in contact with Deramores and they are going to replace the whole order of yarn and hopefully it will arrive tomorrow. I'm going to just frog back to the end of the first ball and cross my fingers that the new yarn matches better. If not then I'm going to have to completely restart :-(
Suppose I'd better do the heels for Mr Soaring Sheep's socks. Almost finished now :-)