I just installed a Philips Hue Outdoor Motion Sensor. It uses AA batteries, has a nice mounting kit, and provides Motion, Temperature, and Illuminance data. It is directly paired to my Hubitat hub. The Hubitat driver has some pretty nice features for setting the sensitivity of the motion sensing aspect. I have noticed that it does pick up our cars in the driveway, even after dialing the sensitivity to be pretty low...but as you stated, a few false motion reports aren't the end of the world. It does pick up people as they walk up to our front door very reliably.
I'm not sure if it's available in UK frequencies
I can't find it on a UK site. It's got wires. What am I meant to do with them 
You could tie it into a spotlight.... Or mains power it direct.. Here is what I'm doing with it but using FOSCAM camera
Motion sensors like this are pretty much all PIR sensors. So when the "see" something warm that is different enough from whatever was there before, whether it's a person or a sunlight, they're going to go off--probably more so for sunlight when you're outside since there's a lot of it, it's very warm, and it's probably changing constantly with clouds coming and going, wind and reflections and other objects, and what have you. Putting it in a birdhouse like you did might help a bit, aside from the narrower field of vision it might have (if the entire sensor is inside), but most dedicated "outdoor" sensors have both a narrower field of vision in their lens and the ability to adjust sensitivity--thinking of things like the Hue Outdoor Motion Sensor and the Zooz ZSE29, two standalone units I can think of. At least one of these also still recommends placing them in a semi-protected area.
But, like you, I've also used a Sonoff motion sensor outside. I figure it's cheap enough that I don't care if it breaks or my experiment fails. Mine are both somewhat protected from direct sunlight but do still have times when they send off event after event. I deal with that by using three sensors in this area (not all Sonoff, in my case) and requiring at least 2 to become active within a few seconds, coupled with Zone Motion Controller and a "false motion reduction" zone, in order for the zone sensor to become active. This works decently well for me though occasionally gets a few false positives for an hour or two certain times of the year. I could probably add more sensors to further reduce this or maybe require all three, but it's good enough for me as-is.
(FWIW, the Sonoff sensors to seem particularly more sensitive than others I've used, perhaps because of their large, protruding lens. My Z-Wave sensors outside go off less...sometimes so much less that they barely see me inside our outside, but that's another story. Ha.)
Another suggestion I've seen is to use beam-break sensors instead. I'm not aware of any that integrate directly into Hubitat, but any that just open/close a circuit when the beam breaks would work. If it's a very specific area, like a front porch, a pressure mat that does the same might work, too--something others have done on couches wired up to a Z-Wave contact sensor Hubitat-integrated alarm planel, etc.
You could potentially use the passive contact output of a beam break system to wire into a contact sensor with external terminals on it...(there are a coupole of zigbee and z-wave ones like that) That's predicate on the beam break sensor having the proper output wires of course
@csteele thank you for the idea, I am going to try the Motion Zone 'False Motion Reduction' mode.
I have collected so many motion sensors in the last year so that I can put two motion sensors (of a different make/model) in every room now... The only problem that I may have is with my better half - she already started complaining when noticed new sensors placed everywhere in the house... But hopefully, I will handle it! : )
I did actually consider putting a thread across the access space with a contact sensor so the contact would be broken if someone tries to walk through it
It sent two falses in a row again. This is bothering me. What's happening to the active = true events that are meant to be happening in between false?
Then when the thread is broken, a switch turns off, de-activating a solenoid causing an anvil to be released and drop on the intruders head!!!!!!! 
You have me thinking now............................ 
Two guesses: if you're using a custom driver, maybe it's doing something odd. More likely, since this is outside, maybe you're at the fringe of your range and some of the messages it's sending to the hub are getting lost.
I think you could be right. There should be a repeater in range as the crow flies but in that particular place the signal would have to travel obliquely within a brick wall (crows usually don't fly in walls). I'll try putting it opposite a window. Though when the Hue was in that exact place it didn't miss a thing
I will also try switching back to the generic zigbee driver
Well, I extended the roof on the birdhouse so it couldn't see any leaves moving and the sun doesn't shine on it. No difference. I've turned it to face the wall. Some reduction in alerts but still not helpful.
I think this sonoff is probably only useful for living in a dark closet to automatically turn on its light.
ACME makes one of those
Yes but I hear it only works on coyotes
Is it the Sonoff SNZB-03?
I have given up with these. Too many false motion alerts.
I even put one in a cardboard box. Completely sealed.
Still got motion detection every couple of days.
Tried new batteries and tweaking the battery contacts to make them tighter on the battery. No joy.
Sold them all on ebay.
Ah...
I am using the SNZB-03 in our "powder room" (WC) and have not experienced any false positives yet 
. I am using a C5 for my Zigbee devices but I don't think that makes any difference.
