When are 'essential criteria' essential?

My post was recently terminated during a restructuring process. People 'at risk' of redundancy were invited to apply for available posts, but only within your own 'pool' or department initially. I applied for 2 posts in my pool, one of which was very similar to a job I had previously done for 10 years at the same place, but had been moved out of in 2006. I was rejected for both posts on the grounds that I did not meet the essential criteria as stated in the job descriptions, though they did interview me presumably knowing that. For the post similar to my previous job, I was the only applicant, but was still deemed unsuitable. I did ask why I was considered suitable years ago, but wasn't now, but both HR and my line manager said they couldn't comment as neither were there at the time (they both joined just over 1 year ago). As there was nothing left in the wider pool (company-wide) by this stage (because of essential criteria), I had no option but to take voluntary severance, which they put back on the table and I had to sign a compromise agreement to get it.
I discovered yesterday that this post has since been given to another person who was 'slotted in' at the time of restructuring. This person does not have the qualifications that were deemed 'essential' for the post when I applied for it, and I have had a longer period of service doing the job than they have. This person was not subsequently interviewed for the post, but just given it. I am rather aggrieved about the whole thing. Do I have any grounds for recourse, given that I had to sign a compromise agreement? The whole thing stinks.

No - you signed away your right to recorse as soon as your name was signed on the compromise agreement. If it is any consolation - you probably wouldn't have got it anyway. If you did not meet the essential criteria then the employer is entitled to refuse to appoint. If they later decide to lower their standards because the cannot appoint, they may do so without it overturning the earlier decision. It happens fairly often - often enough to not raise too many eyebrows.

Thanks for the reply. I thought as much. It still stinks though.