A few assumptions have to be made here:
a) You know (As must the police) who the culprit is (As they have the PCs)
b) If the PCs don't turn up any eveidence, electronic or otherwise, then the chances sound slim. Burden of proof in criminal cases is "Beyond all reasonable doubt" and if there's no evidence, then the test that the CPS use of "A realistic chance of conviction" cannot be satisfied. There's no doubt that it's in the 'public interest' to proceed if possible.
There's nothing more you can do, sadly. Many times I've heard the CPS nicknamed the 'Criminal Protection System' and we do the best job possible, but the system often works against us, and with the criminals gettin lesser sentences and knowing how to play the system, sometimes we have to use the limited funding we're given to the maximum effect. Sucky, I know, I really do. Hope it turns out OK.