thankyou all for your quick replies,
I was thinking of just going ahead following the diagonal for the top of the quilt and it's reassuring to know that doesn't always match up with the back and that that's ok! You're probably right that the recipients won't notice that it's off, I was just stressing because it's a gift and i wanted it to be perfect. I think it's a little late in the day to change the backing fabric considering how long it has been taking me to quilt so far so i'll probably just carry on and keep the front straight and neat.
Thanks again for the help, this site is a useful little treasure trove!
x