I'm wondering... by law do mobile units have to place camera signs before their positions - even if they are a mile away?... scraping the bottom of the barrel here 
That is a very technical area. The road in question should have warning signs within 1mile of "safety camera vans", although there is a general "get-out" clause which states something like "except for exceptional circumstances". So if it was a scamera van, and NOT a uniformed police officer - and you have never seen them before or since at that location, then you aint got a leg to stand on. However, if the scamera van is there on a daily basis, and has been so for a notable period, then signs must be displayed. If no signs, then you can prove they are not operating within the ACPO Code of Practice. <hint, log onto the ACPO website, and download the PDF>!

If it is uniformed police officers, the signage issues don't apply, but they are still expected to comply with the ACPO CoP.