How do I approach this situation? I get on well with my supervisor and other people in my lab. However, my postdoc, who I work very closely with, and I just don't get on. We have admitted there is a personality clash and tried to sort it. However, after a while things got worse again. I have tried to talk to my supervisor but he just says 'that is how she is'. I like my project and would rather not quit, but it is getting to the point that I might.

I thought they were there to encourage you not to bring you down. A fine example is when something doesn't work. I have been doing very well in that everything i touch at the moment works perfectly, but when something doesn't work I am told 'that was a waste of time and money' and 'other people in the past managed to get it to work'. This can't go on for much longer. I am very mild-mannered but I will have to start shouting soon. Any advic would be very useful.

Maybe your supervisor hasn't realized just how bad things are? Being "mild-mannered" like you I get into situations where people don't understand that I feel very bad. Have you told him that you're thinking about leaving? If you tell him in a way that shows you've thought a lot about this, perhaps he'll see how serious it is?
I know I'm not much of an adviser, but I really hope everything works out for you.
Good luck!

Thanks Maria. That was very useful. She is going away next week and then taking 6 weeks off in the summer. I'm hoping to do some great work while she's not there, to try and prove to her that I know what I'm doing. Part of the problem, I think, is that I took 2 years away from acadaemia (but stayed within science) and that seems to have put her nose out of joint as i don't think she thinks I'm committed.

could be a personality clash or it could be jealousy. you shouldn't be told it's a waste of time and money. you cannot know whether an experiment is going to work or not before you do it. the point is to make sure you have good reason to do the experiment. remind this person that they are not your supervisor and suggest they go and say these things to your supervisor instead (having already discussed the situation with your supervisor).