You have an AST with or without a written agreement, you should be able to stay a minimum of six months before anybody can give you notice.
If the cheque you were given for your deposit has bounced then you have NOT been repaid.
They do not have a right to change the locks. They do not have a right to harass you in any way.
It sounds, to me, as if you have a case for some hefty damages here, they obviously have not protected your deposit in a scheme during the first fourteen days of your tenancy. That alone means you are able to make a case for getting a penalty of 3x the deposit from them through the courts.
This is assuming you are not a lodger renting a room in the landlords house, in which case, none of the above applies. (except the harassment bit).
I offer no guarantee that anything I say is correct. wysiwyg