Mugford would be 'in contempt of court' if she took payments and or had a 'verbal agreement' with NEWS OF THE WORLD during the trial.
Yes, in fact it could possibly be perjury, depending on what was said at trial.
But the point that sometimes needs to be made is this:
In English criminal evidence, there is no strong adherence to the doctrine of the 'poisoned tree'. Just because evidence, or even a whole prosecution case, is built on corrupted actions, it doesn't (at least, in and of itself) follow that the evidence is inadmissible.
This is in stark contrast to countries like the United States, where criminal defendants have certain legal and constitutional protections and even a well-proved criminal case can be dismissed on the basis of a procedural or technical irregularity. (However, even in the United States, these protections are tempered by pragmatism and exigency and therefore not always adhered to fully or at all. An example would be the Neil Entwistle case in Massachusetts, where early in the inquiry the police patrol officers had conducted an illegal search of Entwistle's home. Entwistle's lawyers tried to have his conviction overturned on that basis, but this argument was rejected).
Of course, I'm not trying to discourage you in undermining Mugford's evidence, as it might lead to something anyway, but personally it wouldn't be my focus. I just don't think she's very important.
Her evidence isn't probative, at least so far as I can see, and I always laugh when I see 'anti-Bamber' commenters cite something she said. Her evidence is a joke. Undermining her achieves nought.