I would phone and have a discussion rather than email.
There are several good tips already on the thread e.g. "I'd love to take the job. I'm really keen to [show how you'd add value] But I can't accept the current pay offer." (Or "The offer that you have sent is lower than I had been led to expect.")
If they begin to fob you off, ask what the criteria are for being paid the higher end of the scale.
E.g. - If they say, "The top end of the scale is paid to someone who has experience and success in the role" if you already have experience and achievements from previous roles, then obviously this should be pointed out at this stage. E.g. Management experience - "My last two roles have involved managing teams of 12-16 staff, so this is not a new area for me". [They should already know this from your applications and interview - that's why you've been offered an interview and the job]
If you want to take the risk, you could put a review into the negotiation "So if I demonstrate that I can successfully (manage/meet targets/etc) the salary can be reviewed at the end of six months?" but beware if after six months they "forget" this. (Get it written into the offer letter!)
I would only have this discussion in person or over the phone - that way you can choose your words carefully and match the tone of the other person. Emails can easily be misunderstood.
And get anything agreed to written down in the offer letter or email. If anything is missing, get back to them. "The three-month / six-month salary review that we agreed has not been included in the letter ..." and make sure that they update the letter.