Even if the job advertised is not the same as the one you did, but is sufficiently close that you might be able to do it, you could still have a case.
This is because your employer should have actively investigated the possibility of suitable alternative employment for you when you were faced with redundancy. It may appear to a tribunal, after looking at the evidence, that this avenue was not properly explored.
However, it is also possible that circumstances have changed since you left. Your former employer could argue that there was no prospect of a suitable alternative job when you were declared redundant, but that, for example, since you left, a new unexpected order has come in.
Your employer should be looking for suitable alternative roles for you right up to your last day of employment.
Remember that tribunal deadlines are very short – just three months.
The first step in any tribunal claim is to contact Acas by submitting an Acas Early Conciliation Notification Form. If you want to claim that your selection from redundancy was unfair, you must submit the Form to Acas within three months (less one day) of your dismissal date, otherwise the tribunal will not be able to consider your claim.
Acas conciliation is free, and it is an opportunity to try to settle the dispute without a tribunal claim. If Acas conciliation fails (or if you or your employer choose not to participate in Acas conciliation), Acas will issue an early conciliation certificate. You will need the number on this certificate to bring your claim.
There is more information in this Acas leaflet: conciliation explained (PDF).
All the deadlines in the employment tribunal are extremely tight and if you miss them, the tribunal is unlikely to consider your claim. Take advice from your union rep and make sure you don’t leave taking any step until the last minute.