check out inventory and deposit back

Tenancy Deposits

Not sure this is the right section but here we go.
Before the tenants (about to serve notice to them) moved in I wrote and we signed a details inventory.
However during the years there have been all sort of nonsense and I can expect the same on CheckOut day.
I am thinking to employ a professional to carry out the task to make it less bias as possible.
Unfortunately i used the RLA standard AST contract and it does not state anything about the Inventory (sic!)

Can I charge the tenants, or go 50% - 50% with them on the cost of such inventory?

There are already wiriting emails saying that I must give them the deposit back NOW hey will no be able to move
As far as I know I have 28 days to give the deposit back. Having said that I do not want to delay their departure and will happily give it back after the Check-Out Inventory.

Can anyone tell me by law what is the timescale for returning the deposit back?

I foresee a lot of troubles/dispute as they are under the illusion that have kept the flat fabulous while that is far from the truth (IE fittings at least 50 nails to hang their photos) but I do not have the time and energy to fight this battle. Hence I will like to pay for a professional and get them to pay....

Any answer most appreciated