So should I tell him, very politely of course, that he had not kept to the deadline and so I could ignore his letter, or is that likely to make him determined to get me some other way. Any advice please.
Yes, it appears that he has missed the deadline. You could tell them where to go, but since there is a definite underpayment of income tax HMRC has other options.
They could issue an assessment for 2007-08 on the basis that they have
discovered that income which ought to have been assessed has not been assessed or that your self assessment was insufficient. There is a protective measure for you, i.e. was the information "made available" in your tax return? TBH, unless you wrote in the white space that you had declared only 50% of the interest from a sole account then the information was not made available. In any case, HMRC may well be able to argue that you were
negligent in completing your return - in which case "made available" does not need to be considered.
A couple of definitions:
Discover - find out, reach an honest conclusion, become aware of etc.
Neglect - not taking care, not doing something that a reasonable man would do, not carefully reading the relevant notes, not including all sources of income etc.
You could wing it and see how far they take it, but it may indeed make them more determined. What is the date of their initial letter, and when did you receive it?