[Not a duplicate account]

wow..really weird situation.You have the right on your side when it comes to self-exclusion, because your account cannot be self-excluded unless you request it. Thankfully you have all the correspondence with them and you can prove that. Are you a profitable player? Are they maybe trying to find excuses to get rid of you and your winnings? Sounds like it. Try to seek legal advice on what you can do to tackle the situation, and maybe consider change your bookie with a more reliable one. 

[William Hill doesn't 'understand' decimal odds]

Quite unlikely the person working at the bookie to be that stupid to not convert the 1.80 decimal to 4/5 fractional. Probably the odds changed by the time you got here. 8/11 is very specific.