Access to properties

Access

I am wondering if anyone can help me as I have received different responses from agents when querying access to my properties.

I have properties where routine inspections are carried out by agents on a regular basis but sometimes the tenants do not respond when the agent tries to make an appointment. The same also tends to happen if I want to visit a property where the tenant has rent arrears. If I give the tenant written notice that I will access the property using my key so the agent can carry out an inspection is this ok?

Also, I am wondering how I would have to go about gaining access to one of my properties for the annual gas safety inspections if tenants fail to keep an appointment as I don't want to be caught out by the law if the next inspection goes over the 12 months due date?

One agent has advised that I should not access the property under any circumstances but another has said if I provide sufficient notice and at least 2 people are there it will be ok.

Any help would be greatly appreciated.