Uk self assesment tax return question?

On the tax return form, when completing employment details it asks you for a start and end date.
I worked through an employment agency during this period. I do not work through them anymore, but am still registered with them. Should I put the last date I worked with them in the 'date employment ended' box? Or leave it empty, since technically I have never left them?

They want the dates so they can work out which Employer paid you during the Tax year ..

If you are still 'employed' (as opposed to claiming Job Seekers Allowance etc) and you were (and could be again) paid via the Agency, then leave it blank.

If you have started working via another Agency (or are claiming JSA etc) then put the date = the day after you last did any paid work with them (plus any holiday pay they gave you).

Is this the company who gave you 2 P60s, your other question? There's no doubt about it, agency work messes up your tax affairs, often because they don't take you off their records when you cease working for them. They take the attitude that you may work for them again at some time.
If you haven't had form P45, you are still technically on their books, and the tax office will still have a record for you at that reference. I would leave the box empty on the Tax Return.
Suggest you go back to agency and ask for form P45 to say you have left them. I would expect them to now show a current date of leaving, not when you last worked for them. The pay/tax shown may be nil, depends if you have been paid by them after 06/04/07. However a P45 showing your actual cessation date with the relevant pay and tax would be acceptable. I guess this was in the previous tax year.