Hi
It would appear that you may have been chosen unfairly for redundancy. The assessment matrix used to decide who goes and who stays should be published to all the people involved before the selection takes place. The fact that they moved the goalposts afterwards suggests a little - erm - manipulation going on here!
You may be able to take your ex employer to Tribunal for unfair dismissal but I suggest that you get some professional support before you do. CAB can be helpful. Or, if you join a union, they will support you. Tribunals these days are indistinguishable from courts of law - fraught with things that could trip you up.
What you have to weigh up though, is the cost of taking the step. You are unlikely to get your job back and if you win your case, the tribunal may very well take into account the payments made to you within the redundancy settlement - more likely if it was enhanced over and above the statutory minimum. You may get some help from your insurance company, will it affect next years premium?
Sorry if this sounds a little negative. You have right on your side, it seems that an injustice has been done. But to right that wrong, you have a lot of work to do, put in a great deal of time and effort and on top of that, it may cost you real money. If you are prepared for that - then go for it! The law may say it supports equality, the reality is that some employers often find a way to get around the law...
Unfortunately, I am usually on the other side of the table, so I can tell you that your ex employer will have prepared their case very thoroughly before going to tribunal; they will also have some very expensive legal assistance. Most likely, they will agree to an 'out of court settlement' days before the hearing. You could shortcut the process by approaching ACAS now. They are very good at interceding and forging an amicable agreement.
Good Luck!