However, all was not lost. Through her twilter contacts, my mother (who has her own blog here) had found a solution: Baby Savers had a post on How To Make Your Own Colour Catching Laundry Sheets.
If any dye from the clothes in your washing bleeds into the water, your homemade colour catcher will "trap" the dye molecules and keep them from bleeding on to the lighter fabrics.It was amazing! You can see the difference yourself in the pictures below - the first is the original, the second shows the colour bleed, and the third shows the final piece. I couldn't believe how white the white had once again become, without fading the other thread colours. The background fabric is a lot lighter than it was originally (obviously - since all the blue had wound up in the water!) but it sill looks fantastic and I'm over the moon.
And because it worked so well, I wanted to share the steps with you all, just in case you have your own colour disaster in the future. These steps are from Baby Savers with a few adjustments/ additions from me.
Making A Colour Catcher
As I said earlier - this worked like a charm and my cross stitch piece was returned to its former (albeit lighter) glory. I'm not sure how it will go on the 15 year old piece, but I'm going to give it a try this week just in case.