I'm not sure what they were charged with doing that the prosecution held was a perversion of the course of justice (and of which they were convicted). It must surely have been something other than not reporting the death, because that was covered under concealing the birth (which confusingly is about the "secret disposition" of a child's dead body - see s60 of the Offences Against the Person Act 1861 - link below).
Nor presumably was it going on the run, given that the police said publicly that they weren't wanted on suspicion of any crime.
Perhaps it was to do with them not answering questions for a while, or possibly lying about something (?), in the police station.
Whatever it was, it would have been mentioned in the arraignment (in actual court), because a defendant has to be told at that point what they are accused of doing that the crown says constituted the specified crime.
www.legislation.gov.uk
If they are acquitted of the two charges they are being retried for, mitigation arguments regarding the PTCOJ offence, of which they were found guilty at the first trial, could be interesting. Although a lot might depend on what happened on the fifth charge - cruelty.