## Note – I have summarised my understanding at the foot of this post in case anyone subsequently coming across this tread just wants the solution and not the journey! ##
Many thanks for the further helpful and constructive responses, particularly from 4M2 - I would never have "clocked" that the C5 5/3/2 extension terminals were retrospectively filtered by the fitment of a front MK4 data socket - as you say very "counter intuitive"!
I am using solid copper wires (four core twisted) so hopefully this will have minimal effect on line quality. Am intending to use a RJ11 socket to terminate the data extension at the modem end (I believe I need to wire pins 3&4 of 6 for modem data connection).
The spare copper twisted pair are there with an eye to the future in case of need (if ultimately forced down the VOIP route) to take a phone feed back from router to current master socket located phone.
Just to provide some further clarification/background.
The setup is at my octogenarian mothers home – change is never welcomed, so the less the better!
Previously (owing to her having a fixed 3 year BT Home Phone Saver type contract with severe early termination penalties) once she belatedly discovered an internet need I created a rather expensive solution based upon a Three 4G Unlimited data SIM. Now that her fixed BT contract has expired its time to move on (both from a cost and latency point of view)
She is in a FTTP enabled area, but ultimately I decided to go down the FTTC route for the following reasons;
Easier to route cabling from front of house to rear (under carpet). No power near existing entrance point so ONT location would be messy, would also require long thicker Ethernet cable run to router.
Potential loss of 1471 service if forced to a VOIP telephony based solution (she is a creature of habit) Also if VOIP the need for a UPS to cover power loss.
Cost - FTTC is currently cheaper than FTTP (we are going with Zen, so supposedly fixed rate) I loathe the BT/Plusnet 3.9% + CPR type “in contract” price rises. When you factor this in, I don’t think Zen are bad value – especially given their better support ratings and supplied quality Fritzbox 7530 router. (I have also gone Zen for this reason - albeit FTTP)
Happily her FTTC cabinet is very close, probably under 120m away, so speeds should be more than acceptable (to provide an idea the BT Openreach checker suggests (VDSL Range B Impacted) i.e. worst case a downstream hand back threshold of 53.5Mbps
RobertoS - there is a phone located at the master socket, but there is also an extension phone hence why (for neatness) I also wanted to use the 5/3/2 cam connector terminals on the front of the 5C (which happily I now can following 4M2's clarification as to the presence of filtering – I realise I could have fitted a stand-alone filter at the other end but it would become messy).
I note your point as regards the A/B connections on the master - at the risk of teaching you how to "suck the proverbial egg", the BT 5C Master socket has a cam type A/B terminal connectors on its reverse for the use of the BT engineer wiring in the exchange line, but there are also A/B cam connector terminals on the reverse of the MK4 faceplate. The former A/B I did not want to touch as its on BT's side of the demarcation line.
Thanks to all for this helpful input, things are falling nicely into place - hopefully full speed ahead come go live on Thursday (29th) just need to chase Zen for details of fixed IP and connection password.
## Summary Below ##
Finally for anyone coming across this thread and in the same position I was a week ago to summarise (hopefully I have this correct!)
Picture of the 5C Master Socket with Mk4 front plate here
Picture of the A/B terminal on reverse of Mk4 front plate here
I purchased my 5C and MK4 faceplate from Amazon here
(Mods if this is not permitted apologies and please remove)
The standard BT 5C socket has a A/B terminal cam type connector on its reverse which is for the use of the BT Engineer to connect the incoming exchange line. This should not be touched by the consumer as its on the BT side of the connection.
On the front of the 5C socket (once any lower face plate is removed) are 5/3/2 cam type extension terminals. These are NOT live when the lower face plate (or retrofitted MK4 data/voice faceplate if fitted) is removed.
If (because you have a VDSL connection) you fit a MK4 data/voice faceplate onto the front of the 5C (you will need to remove any existing standard BT lower connector socket first) the 5/3/2 extension cam terminals on the 5C will become filtered so can be used for voice extensions without the need to any further filtering on the voice side. Do not use for data!
On the reverse of the Mk4 data/voice faceplate there are A/B cam type connector terminals which are unfiltered and can be used as an output to a modem for VDSL. Do not simultaneously wire any voice extensions to this connection as otherwise the voice extensions will require additional separate filters.
Finally obviously there are socket outputs for RJ11 (Data) and standard BT phone on the front of the MK4 data/voice faceplate which can be used as labelled.