Betway Casino - Account closed and balance taken

Hi Ask Gamblers
I want to put complaint in about Betway.
On the 5th May I signed up and deposited £250, taking their £250 welcome bonus. I played on some slots and managed to get a huge win. I completed the wagering on the bonus and withdrew £3812.71.
The next morning I had an email asking for Proof of ID and proof of address- things I thought were verified on sign up. Obviously I was keen to get my money so I sent them over email anyway about 90mins after they asked for them.
2 days later I get the same email asking for the same documents which was odd. So I sent them off again. I then contacted live chat to find out what was going on and they said they'd never got any emails from me at all.
I sent them again for 3rd time and got live chat to confirm they got them- this time they arrive ok in their inbox.
I logged back in on the 11th May to find out what the status of my withdrawal was only to see 'account locked'. Feeling somewhat puzzled I contacted live chat only to be told by them "A business decision was made to close you account and unfortunately this decision cannot be overturned".
So Betway have closed my account for winning after signing up which was nice, but what has really got me annoyed is they went on to say "Your Betway account has been closed by the Senior management team, the funds on the account has been restricted."
After some clarification on their poor English and strange terminology they said basically they were banning me and stealing my money with no reason given!
I have not received any official confirmation of this via email Betway seemed to have made a decision and laughably they think they can do this with no legal comeback.
With your help I'd like to try and get this complaint sorted amicably and get my £3812.71 in my bank where it belongs.
Cheers,
Gaz