As a landlord you are entitled to enter the property in order to carry out any repairs that have been reported by the tenant. You must give your tenant at least 48 hours written notice of your intention to enter the property. However this does not apply if you must carry out an emergency repair.
You may also be able to access the property in order to inspect its condition and state of repair, however again you must have given your tenant at least 48 hours written notice.
There is nothing to stop the tenant and landlord from mutually agreeing more generous rights of access if both parties want to resolve a non-urgent problem more promptly.
It is important to note that if you enter the property more than necessary without giving your tenant advance notice this may be viewed as harassment by the tenant who is within their rights to seek legal redress.
The landlord has no right to use retained keys to enter the property without the tenant’s permission, except in an emergency.