i find car drivers flash a warning but not many bikers                                                                                                                                                                              

It depends on my mood and where the speed check is.  If it's in town in a 30 limit - no I don't have any truck with hooligans whatever they're riding. 

On a national speed limit road on a good stretch ... maybe.  Depends if I like the look of you and providing you're not driving a BMW. 

yep, will always let others know. if someone comes up behind me quick on a motorway and i know theres a copper just in front i will wave them down too.                                                                                                                                                                              

pmsl JPharley like it... must remember that one  - i do a thumbs down sign to the oncoming vehicle, bike or car, (bt must admit if car driver is driving like a prat in a bm or somethin i probs wouldnt help em!!!) several times..... i was just moving my hand occifer   first time i seen the thumbs down sign a boy racer did it to me on me bike coming round a corner and i thought he was giving my cbr400 thumbs down... pmsl then i realised he was warning me of cops!!!                                                                                                                                                                              

The test is to ask a traffic cop (politely!) what he's there for with his speed gun.
If his answer is that he's there to keep people from speeding and make the roads safer for others, then he shouldn't have any grouse about people  warning other road users. After all, they slow down....job done.
If on the other hand, he says 'to catch speeding cars/bikes', then he won't be amused if you scare off his natural prey!                                                                                                                                                                             

 I'll allways flash at bikes, and HGV's,.. but not cars,.. most of em dont have a clue whats up anyway, or they are too busy gawping at some scenery!!  HGV ist's get the thumbs down too,.. that's a deffinate way to tell em somethings up, and if its a mate in a lorry who's coming towards me, and there's enough time to do it safely, a slow down with me arm/hand maybe,.. which ever is the best for the type of threat ahead.                                                                                                                                                                             

Iv just received a notice of pending taxation- i mean speeding penalty.   I always let people know its only fair, no one flashed me and i always back off the gas when flashed so i agree people dont seem to bother these days.   Shame as i would have still had a clean license oh well, its only a matter of time for us all i guess lol                                                                                                                                                                              

Darwen is a bastard for speed cameras though Bob - I've lost count of the times I've been on the A666 and they've either been parked on the top, or just by the chinese, or by the Cem...I got 6 gold stars years ago within a week by Frank thingy on the main road for going past a speed gun at 33. Bastards.

That's unusual.  The police usually (although they're not obliged to) use the 10% of the posted limit +1 mph rule.  It allows for speedo and human error so's as not to alienate the public.

So in a 30 mph limit the police don't usually initiate a prosecution until you're doing 35 mph. 

And here's me with an old school spinnin my index finger to emulate their old twirly flashers haha, hmmmm, me now thinking someone thinks am saying they'rs off their nut! oh dear, though most do get it