A break in the chain of custody of the evidence is a long way from doctoring the evidence.
The chain of evidence is absolutely key to a prosecution. Proceeding with a prosecution knowing that the chain is broken is 1) a waste of public resources 2) trying one on with the judge, court and jury, 3) highly dubious at best, and frankly stinks.
If they genuinely didn't realise the chain was broken then they're idiots; can't they read their own evidence bags? I wonder what the prosecution barrister (or whatever they have north of the border) was doing in preparing for the case. Not paying any attention by the looks of it...
This is such a fundamental aspect of a prosecution that for it to get that far is indicative of a serious problem in the system. The judge should have thrown the book at them. For a start, how many other cases have there been where the evidence chain has been broken but gone un-noticed?