They should start by asking at shops that sell carpet tacks and looking at CCTV images from those shops.
Then they should interview the people that did and if they can't demonstrate a freshly laid carpet with those tacks used, they should be arrested.
I think it's perfectly reasonable for cyclists to demand evidence that CID are getting involved and not just let uniform write it off as an unsolveable crime.
And I don't think magnatom describing it as terrorism is out of order at all - it certainly does amount to terrorism. If protests at the G20 can be described as terrorism, then this certainly can be. If I was cycling down a road and there were carpet tacks causing cyclists to swerve and have punctures then I'd be terrified. I would be experiencing terror. That's terror that's been deliberately created, therefore it's terrorism. The perpetrators should be banged up for 42 days, that'll teach them.