@rik777,
It's good you've let your hair grow out a bit... It has given your hair time to heal since you last dyed it... However, being you currently have a combination of virgin and dyed hair left over, it's going to be a tad more difficult to put in blonde highlights.
I'd definitely go and see a professional. You'll be much happier with the end result. Paying a few extra bucks for beautiful hair is well worth it...
That being said, the colorist will most likely have to do a base color along with your highlights... and will have to strip the colour out of the dyed portion of the hair (for the highlighted sections). This way, you'll have an even base colour as well as even highlights.
From my own experience.. I had dyed my hair a dark brown (which actually turned out black). I wanted to lighten it up with some caramel highlights. I had grown my hair out (as you have) so I had some prominent roots. The colourist chose a base colour to dye my hair (a reddish dark brown)... For highlights, we went with the caramel... To say the least, my hair turned out looking like a tiger...
The highlights were blonde at the roots... and orange at the bottom... and the base colour turned out a little darker than I had expected... However, I left the highlights alone... Within a couple months the ends of the highlights had lightened a bit... They looked much better... I put a light red-brown dye over them later, and they looked great!
But just make sure the colourist knows what they're doing... a good colourist will know how to get the highlights even...
Good luck with your new look
I'm sure it will turn out great!