At least a couple of decades ago I had occasion to consult with Thompsons, now in Churchill House.
https://www.thompsonstradeunion.law/.../wales/cardiff
At least, from the description on their website, it should be the same Thompsons who, at the time, worked on behalf of Unison, one of the country's largest Trade Unions.
You are wise to seek some professional help if they are prepared to advise; these kinds of lawyers will normally offer a free initial consultation. Good Luck!