Do I say what I really want?
Should I say more knowing they may knock me down?
Maybe I should say less as I don't think the interview went well?!

My advice is to try and do the following...

If going through a recruiter say "I would feel more comfortable if you ask my recruiter. He / she knows what I am on and what I am looking for". This helps alleviate any pressure on you. With that said, it's unlikely the interviewer will push you.

What if they do push?

Be honest. Say "I am currently earning XXX. Ideally I would be looking for an increase to justify a move, however for me the role and opportunity to develop is of most importance". No one wants to hear 'it's all about the money!'

So, how much more is an 'increase'?

I'd say 'Well, I'm hoping you can determine what you think I am worth based on my experience, however from my research I think an increase of 10-15% is fair'.

Always try and keep it loose. We see too many people state a firm number. It's very hard to go up / or down after this.

Good luck.