Thanks for the ideas. Gidget I think you might be right about the germ thing. Public toilets can be so dark and dirty. I have now bought a portable toilet set, one of the ones that fits over a normal toilet seat. She wasn't happy about it but she sat for a few seconds. It's a start but I wont force the issue.
Aussienic, I have tried showing her that I'll use the toilets too, but she just thinks it's funny and hands me some toilet paper, lol. Don't worry about your son. We got the first wee by accident and she liked the sound of the water so she kept wanting to go.

I guess for now I'll keep putting her in nappies when we do the shopping. I'll keep offering to take her but wait until she's ready.