Bamboo, a tenant has a legal obligation to provide the landlord or his/her agent with access to the property for the purpose of inspection, or to carry out repairs, as long as sufficient notice has been provided.
Again, it all depends on what is "reasonable". It's not "reasonable" (in fact it's positively suicidal) for a tenant to refuse access to the gasman when he comes to provide the annual gas safety certificate, for example.
By the same token, it is not "reasonable" for a landlord or letting agent to drop in whenever they happen to be passing, or to let themselves into the property - even for essential maintenance - in yoru absence in a situation which is not an emergency.
If any tenant of mine refused an inspection - which I normally carry out every 6 months, and which are as much of an opportunity for them to raise maintenance issues as for me to identify them - I'd be very concerned, to the point that I'd think something unathorised or even illegal was going on.
Trespass is covered by civil law so enforcing it against your landlord is going to be a hell of a job; you're more likely to receive a s.21 notice at the first opportunity.
Like most things in life, a tenancy works best if both parties accept that they have rights and responsibilities towards one another, and a spirit of co-operation and fairness exists between them.