voluntary notice period

I have opted for voluntary redundancy (I have good reason to believe that I would have been made redundant soon anyway).
I was informed of their acceptance of voluntary redundancy and offered the SRP, nothing in addition to that other than few days holiday pay due.
I was informed that I was required to work two months contractual notice.
However, although my post is effectively redundant I was asked to work a two week overlap if they found someone to take on the post!
I work 3 days a week as admin/finance, they almost immediately advertised for a book-keeper for two days a week (at a lower hourly rate).
I was also informed that if they could find someone, and the two week overlap happened before the end of my contractual notice, then I could be released early.

Question 1: as my job was 90% finance ie., book-keeping, is this unfair or constructive dismissal?

Question 2: I had understood that if I was released early I would be given pay in lieu of notice but this is not the case. Can I insist on payment in lieu?

I will add that I feel I was "encouraged" into the voluntary offer and am glad to be leaving earlier but I want what is due to me.

Any advice??

Meant to say I have worked for this company for 11 years.
Am over 42 years of age.