1. Can I lock the tenant out of the property until they pay?

No. If a tenant has been placed in administration then there will be a moratorium in place. This gives a company some breathing space. Rights against the company, such as forfeiture or conducting legal proceedings, can only be pursued with either the consent of the administrator or a court order. As noted last week, changing the locks is likely to forfeit the lease. Unless you intend to forfeit and obtain the necessary permission to do so, you should not change the locks.