Phew! Can't answer all of that in 2000 words.
Redundancy occurs when work ceases to exist in a defined area of the company.
To a certain extent, companies can play tunes on the 'defined area' bit by, for example, putting a group of people into an area, stating that the work done in that area isn't needed any more and declaring redundancies. Unions are wise to that and it probably wouldn't be straightforward in your company.
Clearly company business activity can go up as well as down, so it is perfectly OK to re-recruit new staff after redundancies. However, since the period by which a claim at an Employment Tribunal for unfair dismissal must be made within 3 months of the dismissal occurring, every company that knows what it is doing will not re-recruit staff - either temps or permanents until at least 3 months period has occurred.
You are most unlikely to get your job back on the existing (current) conditions if this ever came about.