please explain where I had not done the same as PT Bear?
You agreed to pay letting & advertising fees. In our case we just agreed to pay the rent for the remainder of the fixed tenancy. We paid the rent in advance and they were legally obligated to return our bond.
Don't every try cover rent or other costs with the bond after you've left. That's not what it's for. Tenants should always behave in a manner so they can expect to get their bond back - even if they're just handing over the last months rent then getting a similar amount back to them. Landlords should always expect to receive rent so they've got the bond to fall back on if the house is a mess.
It was 12 (happily married) years ago, but I vaguely recall we weren't bothered if they didn't find a tenant before the end of the lease. They didn't advertise until after Mrs Bear had moved out.