query about extension cable set up in new fibre installation

[#1]

I’m having fibre installed very soon, and will need an extension cable to be part of the setup.
My current broadband setup has a standard BT phone socket on a wall downstairs, with an extension cable running to an office upstairs, where it connects to a phone line and Netgear router.
I want the new fibre modem to be installed upstairs, close to the router and power sockets - i.e. so the modem is plugged into the end of the ext. cable, and then directly into a Netgear N600 via the WAN port.
Anyone got a similar setup, or any experience of what-goes-where in the installation to confirm that this works in practice?

[#2]

I would suspect they would install the filtered faceplate on your existing NTE5 master socket then run a CAT5 type cable to where you want your modem to be situated.
Running it over an extension phone cable would not be a very good idea for FTTC.
What kind of wiring do you have between the existing master socket and your office ? Is it twisted pair or flat type cable ?

[#3]

And a little more info.
When the engineer fits the filtered faceplate on the master socket, it has a CAT5 (RJ11) type socket which you can plug a standard CAT5e cable into to extend it. The BT Modem uses a 4 way plug (not the normal 8 way) but this still fits in the faceplate socket.
There are a couple of terminals in the filtered faceplate which people have used in the past to patch CAT5e cable into and extend to a patch panel.
The existing faceplate of the NTE5 with the extension wiring then plugs into the bottom of the new filtered faceplate and this automatically filters any extension lines run off from the master socket.

[#4]

Thanks for such a quick response DomS.
The existing extension is via a slim, round cable - so I imagine it has twisted cables inside.
You think the engineer will replace this with a new extension cable so that the fibre broadband quality is not compromised, and then plug that cable into the new fibre modem upstairs?  That sounds fairly straightforward.  Is it the kind of installation they do all the time?

[#5]

Did you request the extension when you ordered FTTC ? It's a free addon but not sure if it can be requested after the order has been placed.
You'd end up running two cables from your master socket - one for the FTTC modem and another for your voice service. Hopefully a member of staff can clarify the extension cable situation.
I have heard that tea and biscuits helps the engineers mood when you ask them to run the extension ......

[#6]

Yes, fortunately the order included the extension kit, just in case.  Didn't ask them to include any tea or biscuits though.

[#7]

Very much depends on the "engineer" you may get. The one who did my initial FTTC install in 2010 was an ex-squaddie who had been given 6 weeks training. The job he left was a disgrace. I moved all the equipment etc. minutes after he left.
When I was having trouble with my phone line recently, the engineer who called asked why the original bloke hadn't just moved the master socket to a more suitable location as part of the install. He said there is no charge for this.