my contract says: "the company may extend the duration of your probationary period for such period as it considers appropriate by giving you notice in writing before the expiry of the probationary period." Although I did receive this letter, and did agree to the extension, it was still dated 8 days after the expiry of the original probationary period

I was employed with a 6 month probationary period. 10 days after the expiration of this 6 month period, I was sent a letter to notify me that due to performance issues this period had been extended for a further 3 months. At the time I agreed to the extension and carried on with my job. One month later I was called to a meeting with my line manager and HR Manager and told that y probationary period had been unsuccessful and was asked to leave my position with immediate effect and they would pay me the one week notice period as outlined in my contract. As my probationary period had expired when they informed me that they were extending it, am I entitled to the month long notice period pay that is outlined in my contract as I had been with the company for more than the 27 weeks it states is needed for a 1 month notice period? I know that I agreed to the extension, but the letter I have notifying me of it is dated after the 6 months was up.