tile sealer

For whitening grout, I recommend first trying a solution of Oxyclean which is gentle and fairly effective when used in strong concentration. I use it on marble shower before resealing.

E76,

If you had the time or inclination you can use a grout stain from Laticrete. It is a little tedious but the result is great. Not only can you stain your grout but it acts like an epoxy, it will not crack and should not stain again.

Thank you both.

Primer - is this something i should attempt myself or leave to a professional? i'm notoriously un-handy...

That you can do your self. With mosaic's you might want to do some each day until finished. It is real easy to do.