A. Dear Mr. Murray,
Certainly your situation is unpleasant, but it does have redress. What strikes me the most, however, is that no completion date was fixed in the contract, although if we think that the ´gestor´ was representing the buyers, it may have been part of a pre-conceived plan to attempt to cunningly access ownership of a property without paying the full purchase price: if they thought they could achieve this, I can only describe it as wishful thinking.
In my opinion, the best way forward, after having the buyers lived in the property for almost two years, is to send them a notarised notice/letter, where you give them a period of time to complete the purchase and pay the balance, informing them that not paying up the balance would amount to a contractual rescission with loss of 25% paid up to date. A month is a reasonable period.
I would add in the letter that the purchase price is to be increased in the rental payments which you are entitled to by virtue of a verbal rental agreement, as they are enjoying the property whilst you do not have possession and you are losing the interests of the capital you should have already received. This should put you in a stronger negotiation position.
The worst case scenario is that you end up in Court, with the judge fixing a date for the buyers to pay the balance and deciding on whether a verbal rental agreement was reached or on the contrary they were given unconditional access to the property, until they completed the purchase.
The fact that the contract does not specifically mention penalties does not mean that you do not have the right to them, it would be down to the judge to decide what is reasonable in view of the circumstances.
You should seek legal advice and take action as soon as possible.