Dear Butterflywings,
I am assuming you have been the subject of an enquiry, and you are now close to concluding it with the Inspector?
In the words of the Irishman after being asked for directions 'if i was going there i wouldn't start from here'.
You can get guidance on suitable letters from the Inland Revenue website
http://www.inlandrevenue.gov.uk, go to 'practioner zone' and use the search facility under using 'manuals'.
What you don't say is why the additional tax is due, your offer should take into account factors such as:-
Was it your fault?
was it a simple error or deliberate omission?
Did you tell HMIT, or did they pick it up?
Can you claim new expenses or reliefs now?
It might be worth seeking professional advice, it might save you money, at least the initial consultation should be free, and could give you pointers.
Mike