Where a tanancy has been entered into on the basis of being managed by an agent and the tenancy is worded as such, is it a breach of the contract for the landloed to change or dismiss the agent?

Any well-worded agency agreement should contain termination provisions.

If it does not, then my view is that the law will imply a right to terminate with reasonable notice. There is some caselaw on this but cannot recall the cases from memory (you'd need to search through any recent Letting Update articles on agency)

The only obvious case where I can see potential for breach of contract is where the landlord does not adhere to any agreed notice period for termination, or where there are agency charges outstanding or unpaid.
As a general principle any principal(ie the person granting the agency powers to the agent) should have the right to dismiss his agent.

The tenancy agreement is a conract between the landlord and the tenant. The agent is named in the tenancy agreement because he is able to stand in the shoes of the landlord when it comes to fulfilling the landlord's obligations and the tenant is effectively informed of this through the tenancy agreement. What relationship you have with your landlord should be documented in the agency agreement. if there is not one then only a verbal contract can exist. What is expected would be down to what was verbally agreed and failing this what would be reasonable. It would be reasonable for the landlord to dismiss the landlord at reasonable notice (say 1 month) and the agent is entitled to reasonable remuneration for work carried out with the landlord's consent and for reimbursement of expenses incurred with the landlord's consent (implied consent for things like the EPC and Gas record as they are a must)