Update:
I bought
this one.
It is perhaps like buying some illicit substance
It came from Hong Kong, with fake paperwork describing it as some kind of audio mixer and no manuals. After a couple of emails I got a 2-page PDF from the UK end of the operation, which contains nothing of interest except suggesting that the output antenna should be mounted some distance from the booster - but no cable is supplied for the output (rod) antenna whose connector screws directly onto the booster.
My guess is that these products are illegal to sell in the UK and/or illegal to export from wherever they are made, hence the cloak and dagger stuff. Still... who cares if they work.
It does work, but not much. If you are within say 5m of the output antenna, you get a strong signal, but even behind a single breezeblock wall there is very little.
Also it doesn't make a lot of difference if the mains power supply is plugged it
I am an electronics engineer (though done little RF stuff) and I can see why that is....
It is fairly obvious that unless you live in a bungalow, and mount the output antenna centrally (and pointing up so the radiation goes sideways), it isn't going to work well. A better bet would be to mount it in the loft, with a directional output antenna (not sure where to get those from) radiating downwards.
With the existing rod antenna pointing horizontally, and the unit in the loft, we get 1 bar of signal inside all of the house, whereas previously we got nothing inside.
The input antenna is a directional "plate" type. The booster looks different to the one in the website pic. It looks more like
this one but black.
Despite the limited effectiveness, it is a useful solution (for the £170) to the common problem of rubbish GSM signal in the countryside.