Ohhh that's pretty poor behaviour Dellboy on the part of your employer, changing the grounds for dismissal is a pretty dodgy thing for them to do. It sounds as if it's a family firm so the chances of getting a fair hearing were rather reduced
See if you can get the charity to speak to you as soon as possible. Finding an engaging a a solicitor can be a difficult and expensive process - the reason why the charity exists no doubt!
Good Luck with Everything....
The next step is to think about what you actually might like to do in the future? PA has precipitated a work change, whether it was correctly done or not, maybe it'll lay you a little golden egg but don't let the process of pursuing your employers cloud main issue. What do you want to do next? Something that you will be able to continue to do even if the PA does get worse. Retrain, go back to college..... this is an opportunity to do something that may be you always wanted to do? Must be difficult thinking in this vein after what your employer has done to you.... but there is your future life to consider
Kate
