I would try the purple shampoo - but also I'd mix it up a bit - sometimes the fading colour is linked to shampoo / conditioner build up. If you change brands around maybe once a week or so, that may help. Hair sounds nice, btw.
i do like the colours and its all very subtle....i do love my hairdresser and i give her free reign to do what she likes, i go to the mark hill salon. not cheap (but actually not that pricey either - comparable to local salons) but nor do i look it !! despite being a brunette who should be blonde...it never looks tacky, but it just fades fast and wants to go "warmer" so goes brassy.
will try a purple shampoo once or twice a week - anyone got any suggestions?
Vicarina, I used the 'go blonder' shampoo/ conditioner and found my hair took on a distinctly yellowy orange tone! I use a colour restore one now (in iced blonde I think) once a week and leave it on for the 25 minutes it states. Gets rid of the warm tones and colour looks lovely and bright again. Sorry I can't link (on phone!) but I got it from Boots, in the hair colour section.
I have the exact same issues Vicar! I've given up having several colours now and just have bleach highlights. Even they do go orangey after a while though. My hairdresser's advice was to get them done more frequently but it's so bloody expensive!
I think product build-up can cause brassiness too, so I use a clarifying shampoo weekly and try to avoid silicones during the week, saving them for when I straighten it at the weekend. I did have a purple shampoo before - well a dark bright blue one - it was a Loreal one in a silver bottle from hairdresser, they'll advise you best.