There's a chap in the UK putting a co-axial helicopter together, from scratch, of his own design ... https://www.youtube.com/@Ben-Dixey/videos

Looks like a home-made gyrocopter or similar ?

I would not go near that in a million years !, and definitely would not fly it !!

e.g. tail rotor driven by a long rubber belt.........

You'll kill yourself.

That cardboard box in the corner indicates a worrying potential component source ! ;)

Joking apart; things such as the plastic fuel pipe and plastic fuel filters with joints held together with ty-wraps, (aka zip ties); pieces of aluminium angle, but what type of aluminium - what alloy, i.e what strength and resilience does it have in an aircraft environment - or is it just from B&Q, (no disrespect to B&Q). Ditto, what is the specification of all the bolts ?

You simply don't know, so honestly, I would run a mile from that and NEVER fly yourself in it. You could possibly use it for parts to make a large radio controlled model of some sort, but only pay a few hundred for it at most.

I remember reading an accident report where a similar craft had used plastic fuel pipe which had become compromised by shrinking in the lower temperatures aloft, causing fuel starvation and a serious problem.

Just don't go there.

Personally, I'd put the thermometers on the bottom engine as, like multi-row radials, getting cooling air to the back of the engine (bottom in this case) can be difficult, especially as all the cylinder heads are aligned. It looks like the engines are connected to the rotor shaft via chains; if one engine fails, does the shaft lock up or are there free wheeling devices? There was a guy, Manolis IIRC, who used to post on F1 Technical about his 2 stroke flying pogo stick; this actually looks safer.

You know, its a jolly good job the early aviation pioneers didn't worry about the kind of small details you lot are worried about, if they had aviation would never have progressed beyond Montgolfier 😊!

Right, so you would be happy to strap into this and take off in it ?

Winemaker: those look like centrifugal clutches on the engines driving the chains. So if an engine failed it would probably disconnect itself from the output shaft.

But even so...............

If I was an early aviation pioneer - probably.

I hope to hell, 1) you're well insured, 2) you have top cover medical insurance and 3) you have bigger conjones than me ! Good luck, I mean it

And did they?

A very thrilling 3.5 minutes followed by a tragic ending.

Why not?

No freewheel between drive and rotor? I can't see one.

It's also fixed pitch, there is no cyclic or collective control.

Is it? I see a collectiv and some flimsy cables running up the mast. What I don't see is a cyclic. I think that is more relevant. Probably someone needed the broom stick.
I don't know, why that thing does not autorotate.

There are some photos of a different project here: Ultralight G 1 Helicopter : ULTRALIGHT G 1 HELICOPTER A complete G1
It looks like a direct drive from engine(s) to rotor, without any option to adjust cyclic other than shifting your weight around. You would need some sort of clutch/freewheel option to allow autorotation. Mind you, there will not be a lot of energy in that rotor disc anyway and you would also need a collective control to enable you to arrest the downward motion. I'm still not sure how any kind of control is applied to those blades.