There is no legal definition of reasonable, because what is reasonable in one case is not in another.
However I would agree that having got a man out and done a diagnosis in what would seem to be a reasonable time, the agent sitting on his hands for four days at least is perfectly unreasonable.
Not sure where your seven days clause came from, is it mentioned in your rental agreement?
The best to go about it is put in writing what has happened thus far, including the periods of delays between key events, state that you do not consider this reasonable, and thus that the landlord would appear in breach of the tenancy agreement. Then state that you intend to take reasonable measures to provide minimum facilities for yourself for which you expect reimbursement, these may include hiring in portable heaters etc.
See what then happens before invoking it.
As always, you should be using you AST agreement wording to guide your actions.