Speak to the sergeant and leave them in no doubt that failure to return your bike tomorrow will result in a formal complaint.
You've supplied evidence it's your bike, the officer dealing accepts it's your bike and they know who you are and where you live (and have no doubt already checked you out through the system). There can be no reasonable excuse not to return your bike now.
It might also be time to mention involving the IPCC.
I agree with every part of this except the IPCC bit. They genuinely wouldn't be interested due to the level of the issue and would send it back to a local complaint, so I'd replace that with classic's suggestion of a complaint to the inspector.
"I have no doubt it's yours but I need to give the other party time to respond" is just frankly, ridiculous.
I'm hoping your meeting with the sergeant goes well and resolves it.
As for "on nights so can't release it" - not true. They couldn't access the property office themselves, that's true - but they could email the office authorizing release, and you could then make direct contact with the property office yourself and arrange to pick it up - I've done that many times.
It's getting silly now, and unlike most of the things on here when people moan about the police, this one is genuinely starting to get embarrassing...