I have virtually zero experience of these - can any sage old soul tell me:
What the advantages / disadvantages are?
Are they legal on steer axle of UK registered trucks and if so, are there any restrictions?
Ta very much 
Trilex Wheels, are these the things JJ ?
You used to see them a lot in Germany and the rest of Europe, but rarely over here. I don’t see many with them on these days, just the odd old wagon and drag with them on all axles.
I’ve got no experience of them, so don’t know the good or bad points of them.
The obvious advantage though, is ease of changing tyres.
As to legality, as they still seem to be being made, they will have to be type approved, so they’ll be legal throughout Europe.
When I was in Germany used to see the Iranians run this type of wheel if you see one in the UK just ask them they are on the whole approchable chaps.
you still see them a lot on austrian,and swiss
lorrys and as posted those from middle-east
very easy to change and repair,which makes
life easyer when in those places where one
does not get 24/7 recovery-tyre service
Donkeys years ago I subbed on P&O (United States Lines) out of Felixstowe. USL supplied American built trailers (Skels) so the boxes were already to leave the port (no waiting to drop one off and pick up). 90% of these skels had Trilex wheels, they used to look like they were always loose though. Older trucks/trailers here in the colonies have Trilex as well.
That’s it I got nothing more 
They are an absolute ##### pain. The cleats need tightening every five minutes.Some cleats fit the trailer & some the truck. When they get loose if you brake hard the central wheel stops but the rim keeps on rolling & then the air valve gets cut off ;result flat tyre. Then of course a few inches has been shaved off the the central star which will need replacing ( v. expensive ) because the cleats will never be tight. Its true its easy to change a tyre ; the rim is in 3 parts & come apart with a special lever .They can at times run oval. So , for DIY enthusiasts its an absorbing hobby but for a trucker its the pits. The other problem is that the spare wheel carrier is only desighned to carry a rim with a tyre attached .If you have a blow out & the trye is a mess you are left three pieces of metal to store somewhere.To be avoided at all costs. 
Same as harry, and for the reasons he gave.
I’d agree that it’s very easy to change a tyre, but that’s about the only advantage.
IMHO, the disadvantages far outweigh the advantages.
When I worked in Italy, all our trailers had Trilex wheels. 
cheers chaps, maybe give them a miss then - if the only advantage is when you’ve shagged a £ 300 tyre 
for anyone interested - basically volvo won’t fit the only size of rims available in the trilex format, the 20" won’t stand the STGO loadings we need, and the 24" would foul the wheelarches - bang goes my bit of bling then 
Had a lot of experience with Trilex wheels many moons ago when i was a mere tyre fitter, JWQ Tpt had a load of Fiats with the bloody things 
Just a word of caution when removing the wheels . . .make sure you tap the wedges before totally removing the wheelnuts because they can get really tight and have been known to shoot off like a bullet ,resulting in a broken shin bone . . or worse!!
Had them on the FIATS we used to have on Chapman and Ball.In fact I used to be foreman fitter at the FIAT agent that C & B bought their FIATS from before I went driving for them.
Never had a problem with them.Don’t require torquing up as much as the 10 stud,and there are only 6 studs anyway.
VERY important to pull the studs up a bit at a time as the wheel can go on off square if one stud is tightened too much too soon.
As Suedehead said,careful when taking them off!
When I was in the M/E the Arabs had the big bonneted Mercs that had 24 inch Trilex wheels on 10 stud adaptors,so you could either replace one Trilex wheel with another,or take the adaptor off and fit a 10 stud wheel!