. . . I have the standard Openreach Filter supplied by BT (not sure which MK this is) . . .
That is the original Mk 1
, which was superseded by the Mk 2
, which was superseded by the Mk 3
-- all for use with the NTE5/A.
You will come across long discussions about "which version is best" when the measured criterion is synchronisation speed (or throughput speed).
I would recommend the Mk 3
SSFP, for the NTE5/A, or the Mk 4
, for the NTE5/C.
Every device will have a measurable insertion loss when connected in circuit. For a zero insertion loss do not use a SSFP, micro-filter or anything. That recommendation implies that you do not use the circuit for telephony purposes, it is just used as a RF feeder for the broadband signals. (Weaver
is one such person who does not use the basic circuit for telephony . . . the modem is directly connected to the incoming line.)