if the firm is trying to replace you due to age then it is age discrimination. Even if you had sent those emails the firm should have dealt with them in a timely manner and not waited years
You do not need a union to fight your battles and in my experience ( ex acas conciliator) some of the representatives are worse than useless. remember acas does not take sides in any tribunal matter and can offer advice only
I would ask the firm to provide you with written details of what the issues are . Have you had regular appraisals with a written report? if so do they bring any performance issues to light?
If not then you have more ammunition to fight an age discrimination claim - . You could ask the firm to clarify in the absence of performance issues or discussions on bullying if the real reason is due to your sex or age or both
SNP and Greens end coalition deal
Scottish political mess. Is Devolution working?