Our (not in the slightest) illustrious tenants have finally heeded the
court order and left.
Unfortunately the out-of-control offspring have had a whale of a time
with felt tips ...

I'd get the professionals in (no, not The Professionals of CI5 fame). Your
local letting agents will know of some decent ones.

You just have to hope that the retained deposit will cover the cost. If it
doesn't, increase it for the next tenant. Bump the rent to recover the cost
over the next year - No shortage of desperate tenants ATM.

Don't know the details of your case, but there may be some redress
regarding Criminal Damage if they have vandalised your property. You won't
get a penny back of course, but it's a bit of fun.

Al.