Under an AST agreement the landlord is not permitted to evict you on a whim - if you refuse to leave, in order to 'take possession' the landlord must persuade a court to give him a court order.
In the fixed term the landlord must first serve the tenant a 'section 8 notice' with a 'ground for possession' (there are 20).
Were you to refuse or fight it a court would determine whether the landlord may take possession on the ground in the section 8 notice.
That particular clause you are concerned about is common to the AST agreements I've seen. See for example the government's model agreement:
The guidance isn't specific about "illegal, immoral, disorderly or anti-social purposes" but examples elsewhere include prostitution in the property (doing it yourself or allowing it to be done) or it being used to store stolen goods.
I'm just curious to know if there are any laws protecting me as a tenant from the landlord abusing that i.e immoral is certainly subjective and realistically he could find anything he doesn't like immoral?
It is unrealistic to assume the landlord can take possession based on saying anything he doesn't like is immoral.
Do any laws exist to ensure there is a limit on what can be considered reasonable?
Statute isn't specific about what's "reasonable". Ultimately what's reasonable is what the court says is reasonable. You can look at case law.
If not, am I within my rights to ask the landlord to expand on that clause to ensure there is no doubt between the two parties?
You are free to ask the landlord what that clause means and to define it specifically - the landlord is free to do so or walk away from the deal.
Consider that landlords tend to want tenants who will pay on time, keep the property clean and warn them about maintenance problems - I doubt the majority have any interest in their tenants' private lives that the landlord comes to know about unless the landlord anticipates an economic impact.