jimharri"
I would seek advise[/quote]Not so sure about being given a 1st written warning for every different rule you break to be honest. Where I work it's the standard verbal warning said:
you said the session had ended. did your shift then end? if you finish the session at dead on 5 and it was 5.06 you had this discussion, it is out of working hours. if the coat is uniform it is your work attire, but weather permitting, it was not suitable for that day, so you had every right to take it off. no one can force you to keep an item of clothing on (unless it's the police and your planning to run round a playground in your birthday suit).
i'd ring him, ask him to explain why he sacked you, why does he think he is right in sacking you and if he acts a dick. tell him you are going to take him to an employment tribunal. all you need is his company number, name and address. that would not be gross misconduct for not taking a waterproof coat off on a day it is not needed. if the coat has a logo on it and he wants his logo to be seen, then as suitable attire, he should of provided you with a t-shirt to wear aswell. sounds a bit of a nob.