Although we have Sky we also have an aerial that goes into the RF input of the Sky box then out of the RF1 and up to a distribution unit in the loft where freeview can be used by other TVs in the house – although even then some channels drop out.
If I connect the cable from the aerial directly to the cable that goes up the distribution unit (so cutting out the Sky box) I can only get a signal if a TV is connected directly to the cable from the aerial – not on any of the others after the distribution unit.
So is the signal so weak that the Sky box is boosting it so it works all over the house or is there something I’m missing?

I am guessing the signal from the Sky dish is in a “coded satellite” format and that the Sky Box is decoding it before sending it to the RF out ?
Just re-read your post any my theory can’t be correct if you are getting a signal when connected direct to the TV

What distribution unit is it?

The RF2 out on the Sky box also outputs 9V [if req] to power distribution amps – not sure if RF1 does, my memory is failing 🙂

I’m pretty sure splitting the signal makes it weaker. As to how much I don’t know. worst case if you have 4 TV’s on the splitter it could be as little as 1/4 as strong as the input. If however you have a weak signal to start with splitting it to 2 or more TV’s could make it so week it is no good for anything.

How is your ariel? Old or now big or small (a newer bigger ariel could get you a better signal int he first place)
what is the cable like? if the cable is old and damaged that will mess with the signal
Is your splitter box powered? some are and act as boosters to the additional outputs.
Lastly is the ariel pointing in the correct direction for the best signal?

I’m pretty sure splitting the signal makes it weaker. As to how much I don’t know. worst case if you have 4 TV’s on the splitter it could be as little as 1/4 as strong as the input. If however you have a weak signal to start with splitting it to 2 or more TV’s could make it so week it is no good for anything.

Correct, if you passively split the signal into two, you half the power + a bit of insertion loss, so worse than half
I still reckon his Sky box is powering the amp though, hence it doesn’t work without it in the loop

Distribution unit is powered (although I should check it’s still working!).
Aerial is at least 15 years old as I’ve never put one up at this house – may be time for a new one. I’m sure it all worked years ago.

As it’s a active splitter, you shouldn’t lose enough power to drop the signal off as they usuall have something like 6db gain on each port

The RF2 out on the Sky box also outputs 9V [if req] to power distribution amps – not sure if RF1 does, my memory is failing

RF2 outputs a voltage to drive things like a Magic Eye, but it’s disabled by default. RF1 does not, at all.

So is the signal so weak that the Sky box is boosting it so it works all over the house or is there something I’m missing?

Purely speculation, but it sounds like the Sky box is acting like a repeater. I’d no idea that it did that (and it’s frankly surprising if it does).

Are the connections on the cables good? Ie, are they mating in a manner which makes good connections on the box but shit ones with each other? I’ve seen some inordinately bad RF cable connections over the years.

There’s going to be some gain through the Sky modulator purely because of how it works.
VCRs used to have quite a bit of gain but it’s not generally a good thing and as components have evolved, most of the gain on throughput modulators has been eliminated.

It’s possible that the power of the aerial is so marginal that removing the Skybox takes below what’s needed.
Of course it could always be the other way where the power is too much without the attenuation from the skybox. – but I doubt it.