My dd has been very resistant to sitting on the potty. She is 3.0. I was going to give up for a while but decided to order her a Baby Bjorn potty chair and got her to choose the colour. She loves it and actually wants to sit on it! She even asked to have her knickers on.
So while I don't know if this will work now I think it's the best chance of success I have whenever it happens!
I think the mothercare potty was uncomfortable for her - maybe she was too big for it? A step in the right direction anyway.....