Can the bailiff lawfully enter the property using the landlords key?

No

Enforcement regulations are silent on using a landlords key to enter a debtor home, case law provides guidance on whether this mode of entry is legal.

The use of a landlord's key to gain entry is unlawful.^[1]

If the key is found but is not in the lock, the entry would be unlawful.^[2]

A bailiff may not encourage a third party, such as workmen, or another occupant, to allow the bailiff access to a property, access by this means renders everything that follows invalid.^[3]

^{[1]Miller v Curry [1893]
[2] Welch v Krakovsky [1919]
[3] Nash v Lucas [1867] 2 QB 590}