Reduction of Salary - help-advice please

Hello. appreciate this maybe a little long winded but here goes.
Last month, Mid August, received a letter from employers stating company in "financial difficulty" etc, and serving me a one month notice,and that a revised sales target will be needed. Should this target not be achieved then employment will cease at end of that month.
One month later, another letter received stating that sales target not achieved, but the company wishes to offer continued employment under following terms,:
Reduction in basic salary by 50% but give me an opportunity to top up to full salary with commission on achievement of revised target. Notice period to roll over by 30days. Again, failure to hit target means employment will cease on certain date.

Ok. Question.
Just received Septembers payment, in fact on the 24th. The company has already deducted 40% of gross salary. I have queried this, and the director has said as the notice period started 3/4 of the way through a month.
At the time of receiving this notification, I asked for clarification, if this meant Octobers salary and Not septemebers. the director responded by saying, correct, its for the month of October.

How can the company deduct 40% (why not 50%) from septembers, when agreement was given that it was to be octobers?

Anyone any suggestions.Is this legal?

SORRY NEEDED TO EDIT THE FINAL PART.
Ok. Question.
Just received Septembers salary payment, in fact on the 24th Sep. The company has already deducted 40% of my normal gross salary. I have queried this, and the director has said that as the notice period started 3/4 of the way through a month, your salary has been reduced to take into account the 50% deduction.
At the time of receiving this letter stating offer of 50%, I asked for clarification, if this meant that Octobers salary would be reduced and Not septembers, the director responded by saying, correct, its for the month of October and your septembers payment will not be affected.

How can the company deduct 40% (why not 50%) from septembers, when agreement was given that it was to be octobers salary?

UPDATE: Apparently the director concerned was ill informed when he presented me the offer letter. HE SHOULD HAVE STATED THAT SEPTEMBERS salary will be affected. (Funny that)

Anyone any suggestions.Is this legal?

Thanks for checking.

You employer cannot lawfully reduce your pay without your signed consent to the reduction (this is covered by section 13 of the Employment Rights Act 1996). If you have signed to accept this with effect from 1 October, the September reduction is unlawful and you should ask for the money to be paid back. If your employer refuses you are able to bring a claim to the Employment Tribuanl for unlawful deduction from wages (as per s23 of the ERA 1996).

If you have not accepted the reduction at all, you employer will again be acting unlawfully if they impose the reduction.

Hope that helps

Pete