Should an employer sack an architect under JCT Minor Works Building Contract 2009, and in his / her place appoints an unqualified surveyor, can the contract still be considered valid?

Yes, but there are various conditions related to this. Under the JCT Standard Building Contract, article 3 explicitly states that this re-nomination must occur within 21 days of the architect’s dismissal. A contractor has a period of a week to object to this decision, and can be referred to an adjudicator, arbitrator or judge should this be required.