On December 18, 2000 my daughter Kirsty MacColl took her sons, Jamie and Louis, on their first scuba diving expedition, in a marine nature reserve off Cozumel in Mexico. Louis told me later of their excitement as they surfaced after the first dive: “I looked round and smiled at mummy and I said ‘Wow!’ and she smiled back and said ‘Great!’.”

Then it was finished. Kirsty was struck by the propeller of the Percalito, a speedboat owned by one of the wealthiest men in Mexico, and died almost instantly. Her last action was to push Louis out of harm’s way.

My beautiful talented daughter, who was coming into her own again personally and musically after a difficult time, was dead at 41. For her family the loss is incomparable. The bravery of her boys, now 17 and 19, in remaking their lives is heartbreaking.

When I cry now my tears are those of anger. Although my daughter’s injuries were horrendous, the speedboat should not have been in an area designated for divers and their support boats. Yet its owner, Guillermo Gonzalez Nova, head of one of Mexico’s biggest supermarket chains and who was on board at the time with members of his family, refuses to see me or even to answer the letter that I wrote to him earlier this year. There has been not one word of regret from him and there are questions that I want him to answer.

There was a trial. Juan Jose Cen Yam, a 28-year-old deck hand, was convicted of culpable homicide and sentenced to 34 months in jail — although he was allowed to pay a fine, a paltry £61, instead of serving his sentence. The court did not hear witness evidence, published in the press last week, that it was not Cen Yam who had been steering the boat.

I am determined that there will be justice for Kirsty. I couldn’t do it on my own. Fred Shortland, who met Kirsty at her last British concert, has offered to help. He runs Casa Alianza, an organisation that helps street children in Latin America, and he guided me through the difficult early encounters with the Foreign Office — which is now being wonderfully supportive.

With my friend John Dalby, I travelled to Mexico in March to see the site of the accident and to try to find out the truth. I met several government ministers and was received with great sympathy and courtesy, although I was astonished that for some it was the first they had heard of the case.

Dr Gabriel Szekely at the tourism ministry thanked me for coming. “But for the grace of God, it could have been one of my family. What can be done?” he asked.

That’s simple: get people to obey the law. We can’t bring Kirsty back, but at least our work might prevent more terrible accidents like hers. Gonzalez Nova, however, seems to consider himself above the law. People think of Mexican men as tough and macho, but I believe his behaviour cowardly and underhand. He has even claimed that our motive is to get money out of him.

This is contemptible. What we want is justice and we have spent enormous sums of money on our campaign. We had begun by hiring a private investigator from New York. He found more witnesses in Cozumel whose testimony states that it was not Cen Yam who had been driving the boat. The investigator was excellent but expensive.

Then we needed someone who knew Mexican law. Demetrio Guerra, our brilliant Mexican lawyer, has persuaded the federal prosecutor to look at the new case he has built. Any day now he will tell us whether it is strong enough for a new trial. We think it is. Guerra is committed to our cause but he needs to be paid. So far the campaign has cost £200,000.

At first Kirsty’s former husband, music producer Steve Lillywhite, paid the bills, and then Kirsty paid. Can you imagine how galling it is to me that my daughter’s earnings were being used to retrieve evidence of her own terrible death? I spent most of my savings helping to fund the trip to Mexico.

If the federal prosecutor opens a new case, I hope Gonzalez Nova will face a charge of perjury. If the prosecutor refuses? Then it’s battle stations. I will never let it rest.

We will appeal to a higher level if we need to. I am elderly now but my friends say that if anything happens to me they will carry on. They will never let it go.

The Justice for Kirsty campaign has Spanish and American branches and Padi, the diving body, is behind us. More than 8,000 people have sent postcards demanding justice for Kirsty to President Vicente Fox of Mexico.

The warmth of this support is fantastic. However, the stark truth is that if there is no money, there is no campaign. People have suggested benefit concerts but nothing has happened — yet.

As the wife, friend and mother of musicians I know they are not the easiest people to organise — but I hope someone will do something. We cannot stop now. And I think I know what Kirsty would have done.