Timetable

LU don't publish timetables for passengers (except for the north end of the Met).
Timetables obviously exist for staff use, these are known as Working Timetables (WTT) and each one is numbered in sequence from the opening of the line or a major change in the line (e.g. the Jubilee Line numbering restarted when the extension opened).
Working timetables though are classified as internal documents that are not for publication, so in the normal course of events you don't get to see them (although sometimes old ones are available if you know where to look, and other forum members can help you here if you are interested).
These days however you are in luck as you can request copies of the current working timetable using the Freedom of Information Act, but you will only get a hard-copy version. If you want to go down this route, email enquiry.tube@tfl.gov.uk with your request, specifically mentioning the Freedom of Information Act^[1] and include your postal address. Legally they must respond within 20 working days.

[1] Legally you don't have to mention it, but it doesn't cost you anything and it makes it clear.

Yes, I would just ask for the current working timetable for the Northern Line rather than a specific number (unless it's a specific historical one you're interested in of course).

Also, as Alfie says, it's always best to be polite about these. things. There isn't any need to mention the deadline unless they've exceeded it and you're sending a followup.

Interesting to see that TFL can now provide them in electronic form I still wish they could do them in a computer-readable form though so I could do things with them

While I'm here, who is responsible on the day for ignoring and over-ruling the time-table? A quick glance tels me that, Monday to Friday, there should be several Charing Cross southbound trains in the late afternoon and early evening that run all the way to Morden. Frequently there aren't any. Who has the authority to terminate all the SB Charing Cross trains at Kennington, in apparent contravention of the time-table?

Wrong word - failure to understand what a TT is. It is (and can never be more than) only ever a statement of intent barring adverse circumstances.

These circumstances can include train failure, signal failure, passenger action, staff shortages, illness, track failure..............the list goes on.

The word "contravention" implies legal fault and is totally the wrong way to look at it. Believe me, all staff want to run as many trains as they possibly can - it was true before, and is even more so now with the advent of performance targets. Cancellation of trains is only ever done as a last resort, due to the known consequences (overcrowded platforms etc.) of doing so.

Repeated cancellation of the same trains indicates some sort of recurring problem yet to find a solution .