. . . 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Mk 1, which was superseded by the
Mk 2, which was superseded by the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.)