My old job is advertised in the paper!

No. Why should they? Their obligation to you ran out the day your employment was terminated. They didn't need to inform you, and so that can't be discrmiination

I am not naive to think that trade unions are any different to any other employer, but, I would have thought that they would give those who lost there posts the chance to re-apply for them in the knowledge that this is the sort of good practice that trade unions advocate to employers and also the fact that we are experienced in this field of work so therefore would not need to be trained or establish customer relationships.

That may well be true - but what may be good practice or sense is not what is law; and the questions you asked were about whether you had a legal claim for unfair dismissal or discrimination and you do not. It is perhaps unfortunate that you did not see the jobs being advertised, but even if you had, you would only have been entitled to apply in competition against other external candidates. being a former employee would have given you no advantage over anyone else, and equally, may have actually been a disadvantage - I have certainly known of circumstances in which the "known" candidate is judged by what is already known about them (it's harder to "spin" if you are known) whilst others can say what they like about their skills and experience with relative impunity because nobody knows anything other than what they say. Interviews are not ideal methods of selecting people - but equally if they had simply "taken you back" they could have in fact been commiting discrimination by exclusing other candidates.