But Colin and Cressida are not forensic scientists.
This series is a drama not a documentary with input from appropriately qualified people about a serious subject.
The producers know such a drama will pull in audiences and make money.
I can understand Colin Caffell writing a book in the aftermath but I can't understand why he wishes to produce an updated version 3 decades on if not just to make money. Why now? Why did he not wish to get involved when CAL sought him out?
JB is serving a whole life sentence and until such time he can provide compelling new evidence to undermine his conviction he will not be going anywhere so what is the purpose of such a drama?
Jeremy and the CT are regularly in the news bleating on about new evidence - if Colin now wishes to readdress the balance, good on him. He has a right to comment and as he stated, the drama was going ahead anyway, so better to be part of it and have some input.
The drama is the result of CAL's book, but such things take a while to organise - his input is clearly the result of the book so the answer to you question is - he did.