The above is OK if you know the pay bands but interviewees are rarely in a privileged enough position to be able to know all this at this stage (not true in the public sector - that generally make every band available to all and sundry). What about
Dear XXXX
Thank you for your offer of employment as xxxxxxx. I am very enthusiastic about the prospect of joining the company and feel that my knowledge and experience would add much to the organisation.
However the salary that you have offered is less than discussed during our interview. I would like to ask if you will reconsider this aspect of the renumeration package as I feel my skills are worthy of �xyz in the market place.
Yours sincerely,
I think this more direct than Cassa's suggestion - it isn't about 'clarification' - its about stating that you consider you are worth more. It also doesn't say you won't join, but restates that you are very keen to join - but there is a problem...............
I suggest you name a figure but bear in mind they may well come back with a counter-offer - which probably at that point won't be shifted further - so name a figure that you might be happy to move down from.
Don't accept the unchanged offer, but a review after 3 months. say. It gets you hooked in, without the real prospect in the increase after 3 months - its an old trick.