Christopher Key wrote:

Sorry, should have been more clear. As we have SKY+, i'll be taking
a 2nd feed from the LNB down to the lounge, which won't be connected
directly to the loft box. It seemed a little untidy having this
earthed 'via' the SKY box.

Still no need, the 2nd IF feed will be earthed via the body of the LNB
and the 1st IF cable to the loft box.

Moreover inserting low-grade connectors & adaptors (such as a grounding
block or ordinary F back-to-backs) in a digital satellite IF line is not
recommended since some of these devices exhibit very poor impedance
match (poor return loss or high VSWR) toward the top end of the IF band
at ~2 GHz. The reflections caused in the cable lead to signal
degradation (inter-symbol interference) which can sometimes cause
missing channels etc. If you need an in-line F coupler for satellite,
use the Cabelcon F-81HQ type which provides a really good 75 ohm match
from DC to 2 GHz+.

--
Andy