Caz, my advice would be to phone first and explain to the salon what you aren't happy with and see what they offer. (I find the phone less embarassing than doing these things in person). If they don't offer to fix it up for free then I think that dark brown with copper sounds lovely too!

You could try to put a semi-permanent through it yourself as it should take to the lighter sections more quickly and thus tone done the highlights. Depends how willing you are to risk making your hair worse! (I'd do it but then i also let my husband cut my hair!).