+1 for Crazylegs’ appraoch.
Another option is to say that it’s too early to discuss salary. You’re not in a position to know what you would want to ask for, they are not in a position to know what they would want to offer you.
It’s always better to push salary negotiations back until they are ready to make you an offer and you are ready to accept one. Otherwise it’s just a tactic to manage your expectations and try to get you cheaper than they might be willing to pay.
If they realy really press you, take what you’re earning now or what you think is a reasonable figure and add 50%. When you deliver it say
‘Well £x’000s would be nice’ and see what they say.
If they say that the figure is ridiculous then you can either respond with, ‘OK tell me what you think is reasonable?’
Whatever they say next, the important part is to follow up with ‘OK well we can negotiate the package when you’re ready to offer and I’m ready to accept’.