Well, completion of sale is just that. Until that occurs, the
property is not legally yours and the existing owner can rent it out for whatever amount he considers reasonable and rent up to the point of completion accrues to him or her.
Any tenancy agreement signed before completion will be between the existing tenant and the pre-completion owner. You simply take over the tenancy upon completion. Of course your agent might come up with what he or she thinks is a creative way of getting round this legally, but a lot of agents have absolutely no clue what they're doing and may simply get you into trouble. They never admit to not having a clue either which is worse.
You could of course consult a lawyer, but this seems a small matter to be paying a fair bit of money for.
How much time do you have before completion? And how close is the date of the tenancy expiry to the date of completion?
Quite simply, you can and should attempt to discuss this issue with the property owner. By now, you should have their contact details, or at the very least their names with which you can contact them. Perhaps they and the tenants are willing to settle on a rolling contract up till the point of handover while you concurrently are discussing with the tenant a new agreement to begin on the day of the handover.
The existing owner is not likely to want to use the agent to negotiate a contract extension and pay a new commission to the agent if he or she is only going to collect a month or two of rent more.
Cut out the agent altogether in this process. The agent didn't find you this tenant, and the agent has also been paid for securing the original tenant (assuming it is the same agent) so there is no good reason at all for this agent to be trying to get anything out of this other than the satisfaction of having provided good service.
Having someone in the middle simply complicates things. Information gets miscommunicated, and often deliberately withheld. You try to work this out amonsgt 3 parties though an agent, you never know what was said to each and every party because the agent will almost certainly tell everyone what they want to hear.
No other way to say it, but really, issues like these are things you ought to have thought about and discussed with the property vendor before you even paid the 1% option fee.
But really, there is not very much you can enforce in terms of action because quite simply, you don't own the property until completion.
