Hello again Handyman44 (I was "off duty" yesterday!).
Sorry, no criticism implied, I was just trying to obtain max info, to give best help.
(Anyone who self installs a multiswitch - without previous experience of this - is being quite brave!).
If cable inner and outer conductors are touching, anywhere, usually you would get nothing at all (certainly no sat, or digi terrestrial, though still possibly residual (bad!) analogue via induction, depending, etc ....).
If you've used any type of "satellite cable" for the distribution, whichever one, then there should be no problem in actually distributing the sat signals (that's ignoring any additional interference, which might make the sat signals "un-decodable"!). There's a huge difference between sat cable, and "ordinary" cable for UHF tv. Sat frequencies will not "travel" along ordinary tv cable, for more than just a few feet, and obviously you haven't got that problem!
Yes, a "proper" aerial combiner - with bandpass filters built in - will certainly help, if you've got broadband high level r.f. interference (it certainly seems like you have!).
I'm sorry, apologies, but it turns out that - by pure bad luck - you first got a multiswitch with the worst terr-sat separation figure I've ever seen, maybe an old design, normally this should work ok, but of course it's more susceptible to interference problems, should there be any!
(I don't want to say "Labgear multiswitches are bad" - since often they would work, and otherwise Labgear do a good and very useful equimpment range - but other people be warned!).
(My multiswitch guide didn't mention terrestrial signal distribution problems, since that's really a different subject! Normally, even if there's a problem on terrestrial, a multiswitch with "usual" 35-40 dB separation would give fair protection from possible consequent problems, ie, sat amps "overloading" from spurious distortion products).
(Note that, that's terr- to- sat separation, and not the completely different outlet- to- outlet separation, which is probably only 20 - 30 dB!).
Yes, a DAB aerial isn't quite correct for fm band 2, but is still probably better than any receiver built-in aerial, for reducing hiss on analogue fm stereo (19 Khz pilot tone system).
Try keeping sat signals at max level, but attenuating the terr section as much as allowed, should be no noticable difference terr analogue tv, but check all terr digital muxes still work. This should reduce the sat interference problem, but you may have to "compromise" the levels between digi terr and sat.
It's still possible to futher band-limit fm aerial output, by adding a special fm bandpass filter - before the combiner! - although that means even more expense! See:
www.jwhardy.co.uk , go tuned rf components, scroll down (band4/5 bandpass filters commonplace, band 2 are rarer!).
Best would be to identify the interference source! Radio transmitters are usually "bursty", constant interference suggests nearby electrical equipment. Unwanted induction along cable outers is sometimes possible - defeating the screening - but mainly from just the fm aerial suggests it isn't induction.