Update:
On the day of the appointment I had made to correct the mistake, I spent some time explaining what was wrong with my hair, showed a bunch of pictures of what I wanted.Hairdresser said she could see what the problem was and she would correct it.
I was to have some highlights put in first. I showed the colourist some pictures of what I wanted. I have dark blonde hair and wanted an ombre look with very light ends. She said that she would not recommend this for my hair, as it would not be visible enough, as my hair is already very light. (It's actually a mousey blonde and not very light). Instead, she recommended putting 'slices of white' on the sides, covered by the top layer of hair so there would be no roots. I asked her if this would actually be visible with a one-length cut without layers and she said yes. She said "trust me, this will look much better", so I let her get on with it.
When the bleach was washed out it became pretty obvious to me that there were no highlights to speak of. The hairdresser who was to cut my hair said "your colour is looking lovely", presumably to make me less likely to complain about the lack of result. I have noticed that they are more likely to rave about the result when it is not very good, presumably to pre-empt complaints. The colourist, of course, agreed that the result looke "lovely and natural".
I was not too bothered by this, as my main reason for coming was the cut. When she was about halfway through, I told her that the hair still looked longer in the back, as I could see it curling up underneath the shorter hair at the front. She assured me this was not the case (something about it curling over my shoulders - even though the hair in front is not long enough to touch the shoulders!). She said "you'll see when I have dried it off that it is not longer in the back, it just looks that way now".
By the time she had dried it, I needed to get back to my baby as I am breastfeeding and my husband had tried in vain to give a bottle of EBM and was texting me, asking when I would be finished. The hairdresser had also styled it so well that it looked nice despite not being what I had asked for, so I did not complain. But, yeah, it was longer in the back, still.
So here I am, two hair appointments and more than £150 later, and my hair still does not look the way I wanted. I guess a long bob that is all one length is some kind of impossible, unattainable goal. I'm just gonna grow it now, as I have lost hope of ever finding a hairdresser who can follow basic instructions.