There is no specific law or regulation that entitles landlords to keep copies of keys or to insist on being provided with duplicates for a property that has been rented out. However most do retain a set and if appropriate notice has been served on the tenants of the pending visit it is very tempting for landlords to allow themselves in.
There are risks associated with letting yourself in and the reason why most lawyers will advise against doing so are because the tenant could argue that you have breached your covenant to allow the tenant quiet enjoyment of the property. You are also putting yourself in a situation where tenants may claim that you have interfered or stolen their belongings. The only legal option is generally to obtain a court order in the first instance especially for residential premises. Just because the tenant is under an obligation to give you access you have no right to enter if the tenant is opposed to your entry, even if you have a set of keys.
If you are prepared to take the risk of false accusations and the tenant does not indicate to you that they are refusing access after appropriate notice has been served, then you might let yourself into the property to undertake the inspection. If the tenant objects to you gaining access then you must not use violence to gain entry as this is a criminal offence (s.6 Criminal Law Act 1977). Any physical force, even if reasonable, risks being interpreted as violence and so this strategy of entering without express permission from the tenant must be considered a high risk one.
If you are entering commercial premises to carry out an inspection then be wary that any entry on to the property might be interpreted as an act of forfeiture. If the tenant is in arrears of rent and you enter the property other than with the agreement of the tenant, the tenant, looking to shed himself of the liabilities of the lease, may claim you have terminated the lease by reason of the arrears.
TIP – If possible try to video record your visit.
If you would like to discuss any on the points raised above please email me.