Deposit "management" when change of 1 tenant in HMO (via a Deed of assignment)

Tenancy Deposits

I have a student HMO (5 students) let on a single contract (fixed till Sept 21). The corresponding deposit is registered as 1 amount for the property, but with all 5 making the same contribution (20% each))

1. student wishes to leave. They are permitted to but they (we!) need to find a suitable replacement to assign their responsibilities to (using the NRLA Deed of Assignment)

Whilst I'm happy about the Deed & how to manage it, I am not sure about the "management" of the deposit.

The student who is new will be joining the contract part way through, and the house (Inventory)will not be in the same condition as when the original 5 took the house on (we can safely assume oven , fridge, carpets & shower need cleaning)

As such how to I manage the deposits of the 4 who are staying, the 1 student leaving & the 1 student arriving?!

Do I make an agreed deduction from the one leaving, keep the deposit I have, ask the new student to pay the leaving student, get them to sort it out amongst themselves, or what!?

Any direction welcome
Thanks all