A bin wagon will do the round on the same day and time each week (or fortnight) so it may be possible to narrow the time down; many have a tracking device fitted so it is possible to confirm if the wagon is in the area at the time. If the times match with the collection from the flats I would write to the company who have the contract and claim their wagon was seen doing the damage. The damage will contain traces of paint from the other vehicle so that is evidence, if it was a bin wagon, the colour will be that of the lifting equipment (usually orange or yellow).
They should investigate and write back. Believe me there are many such minor accidents and the driver may not even be aware he's done the damage. They will settle even though they are only 3rd party insured, it's not worth the hassle to argue the case.
Rob (working for a waste management company)
Brilliant Blue 156 GTA SportWagon | Q2 | KW V1 Coil-overs | Eibach ARB | Wiecher Strut Bar | Wizard Cat-Back
2009 Fiat Panda 100 HP Stealth Edition