I thought I would update you all - thanks everyone for your advice and warnings etc.! Krum, really interesting about toner, that will be my next step I think.
I will put quite a bit of detail down because there is so much conflicting and vague advice on henna removal and I have searched the web to almost no avail, so if I get it all out here, some other poor soul might find it and get some tips!
Just for background:
My natural hair colour is almost exactly like this, though perhaps slightly warmer.
I'd had highlights, so it was like more like this, though a bit less stripy. I'd like to go back to a cross between this and my natural colour.
6 months of henna and I was pretty much this colour. Utterly gorgeous, right? But I have pale bluish skin, dark blue eyes, almost invisible eyelashes and tiny fair eyebrows, so I started to look like death.
After 2 applications of Colour B4 I was more like this colour which is great if you're born with it and have the freckles etc. but looked shocking!!
I went ahead and bought some John Frieda Precision Mousse in Light Natural Blonde.
I did a strand test like a good girl, and it looked as if it had made quite a significant difference - lighter and much less red.
So with much trepidation, and tugging on a hair about every 5 minutes to check for weakness and breakages, I put it on. I can confirm that it is terrifically easy to use.
My scalp stung a bit, probably because I had already done 2 other strong treatments in the last 3 days, but it wasn't too bad. I rinsed it out, conditioned and dried it, and miraculously and TOTALLY against what everyone says about bleaching over henna that condition of it is absolutely fine. It was shiny, bouncy, even and glossy.
HOWEVER - it's still bloody red!! It has toned it down a great deal, so it's less Ronald McDonald and a much more even natural sort of auburn, almost as light as strawberry blonde but not quite. It's kind of like this
I am hoping, if I use blonding shampoo and conditioners every day, within the next 6 weeks it should tone down/lighten to this sort of thing, which is not too far from my natural hair colour and I could live with, and which won't give too weird a two-tone effect!
So that's my henna removal saga. Ladies and gentlemen: do NOT touch henna unless you are pretty much prepared to be a redhead for the rest of your life! And yes, I include Lush caca!