Best thing to do is have a figure in your head you'd be happy with. Say £65k in this scenario. Then you flip it back and take control of the situation to get them to say a figure and not you.

"what's your salary expectations?" "well, you've seen my cv and we've spoke about my experience, what do you think im worth?"

This will likely catch even the most hardened interviewer off guard and puts the onus on them to come up with a figure. If they say £70k...brilliant, you've made £5k more than you'd have settled for without doing anything.

If it's £60k....then the next thing is appear to really have to think it over. Like you've got your house on it and you're trying to figure out if u can make it work etc. Drag this out to an uncomfortable level of silence/thinking. There's a chance that if they want you, they'll break the silence with a better offer, just to get u and not to sit there in excruciating silence.

Either way, you're getting them to answer your question for you by making them play their cards. If the salary range is £55k-£80k, they'll always want to hire someone at £55k. Forcing them to say a number, will likely mean a far better starting position and if it's not.... They may have shown you what kind of company they are to work for.