By Jon Silverman Legal affairs correspondent

If ever a murder trial has been known for one single piece of testimony, it is that of Sally Clark.

It came from the eminent paediatrician, Professor Sir Roy Meadow, who appeared for the prosecution, as he has done in a number of criminal cases involving the sudden, unexplained deaths of infants.

Asked for his estimation of the likelihood of two siblings dying of so-called " cot death ", Sir Roy rated the chances at one in 73 million.

It is claimed that the tests showed ' lethal' levels of bacterial infection which indicate that he died of natural causes

Since it is forbidden to canvas the opinion of a jury after a trial, it is impossible to be certain of the impact that that view may have had.

But Sally Clark's family and supporters are convinced that it was crucial in convicting her.

Sir Roy's calculation was controversial from the very start.

Indeed, the Royal Statistical Society took the unprecedented step of writing to the Lord Chancellor stating that " there was no statistical basis " for the figure.

And a Professor of Mathematics at Salford University even did a special study to demonstrate the fallacy behind the testimony.

Fresh evidence

However, the judges of the Appeal Court - who will be examining the Sally Clark case for a second time - will be rather more concerned with fresh evidence than opinion.

Campaigners for Mrs Clark say they have it in the form of laboratory tests commissioned by a Home Office pathologist on the second of the babies to die, Harry.

It is claimed that the tests showed "lethal" levels of bacterial infection which indicate that he died of natural causes.

Crucially, the jury was not told of these tests - nor the findings.

Sally Clark maintained her innocence

This fresh evidence is part of the dossier which has been under consideration by the Criminal Cases Review Commission since last November.

And the timing of Tuesday's decision is almost certainly connected to a BBC Radio " File on Four " programme due to be broadcast on the same day which investigates the Sally Clark case in detail and concludes that she is the victim of a miscarriage of justice.

Another mother serving life for murdering her children, Angela Cannings, will be awaiting the outcome of Sally Clark's appeal with impatient interest.

She was convicted in April 2002 of killing two of her babies.

She, too, has always protested her innocence and, as in the Sally Clark case, an expert witness for the Crown was Sir Roy Meadow.