I sympathise with your issue - I've been in a similar position a couple of times.
1. I agree with Tony's comments - it's totally up to you to call the shots. Of course, this is relatively easy to see after you have started engaging with the agents.
2. It’s probably too late to recover your position with your current agent(s). My suggestion would be to choose new agents and be very firm with them from the start what sort of package will be acceptable. In my experience, agents brief their clients carefully on what you earn now and what they believe you will accept. Use this to your advantage.
3. Agents and prospective employers are bound to ask for current package details – do not tell them. Remember, it's not relevant to a new employer (or agent) what you earn now, only what the job is worth. Taxman will tell you that you do NOT need to give out your P45 – complete a P46 and the tax office will sort out your true tax position a couple of months later.
4. There is nothing wrong with saying you're not going to discuss current salary and then state "the range I am looking for is between £x and £y". If prospective employers insist you tell them your current details, ask why they need them. Remind them that you are not looking to move to do the same job for 10% more than now but to do the best job for them you can possibly do, in a new role, new challenges, etc.
5. Any co. is looking to give you the smallest possible salary to get you and keep you motivated for as long as possible and at least until the inertia sets in. It's up to you to demonstrate where that bar lies
6. Consider buying "Perfect Job Interview" by Max Eggert and read the excellent section on salary negotiation at the back. All the rules and how to play the game.