There are two:

T can apply to the court for an order requiring L to repay the deposit to T (or to pay it into an approved scheme). More importantly, the court must order L to pay T a penalty three times the amount of the deposit; 


the normal rule is that L can serve two months’ written notice (in the prescribed form) under s21 HA 1988 to terminate an assured shorthold at the end of a fixed term, or after a fixed term has expired. But, if L has not complied with the tenancy deposit requirements then he cannot serve an s21 notice. So, in practical terms, L will have to remedy the position before he can serve two months’ notice.