Thanks for the advice so far. One final thought, would it be OK to run a long RJ11 (i.e 20m or so from the filtered face plate to the router or better to use a short RJ11 to the router and then a long CAT5 from the router to a switch?

I am not sure that RJ11 is suitable for external runs (I would otherwise buy external grade CAT5)?

RJ11 describes the plug, not the cable. Cat5 cables which are not outdoor rated can degrade with the action of sunlight on the outer PVC coating.

In reply to a post by zebedeee:

RJ11 describes the plug, not the cable.

Thanks. I didn't appreciate that.

So would it make a difference in performance if I run a long exterior grade cat 5 with RJ11 plugs from the master socket in my garage to the router upstairs as opposed to having the router by the master socket and running the cat 5 from this to the router? The only reason I ask is that it would be more convenient to have the router upstairs and would avoid having to buy a 4 switch.

The usual practice is to connect the router close to the ADSL nation filtered faceplate and either wirelessly connect to PC's or use a long ethernet cable.
However, in your case I'm wondering whether Homeplugs would be a better option because you have many PC's. These transmit the ADSL signal via the 240V electric circuits in your house. So you could site the router next to the master socket and connect one of the homeplug adaptors to the router via ethernet cable and plug the adaptor into the nearest electrical socket. The other homeplug adaptor you connect elsewhere in your house into another electrical socket near your PC's and connect via ethernet cable.
I have no experience with these homeplugs but others have said that the connection is solid with no interference or drop-outs, and the connection speeds are much faster than wireless too. Maybe worth searching for Homeplug in Google to learn more - there are several brands and the only negative I see is the cost of the adaptors - around £40 - which has to be balanced against the cost of long ethernet cables and the hassle of drilling etc in your existing proposal.

As the previous poster said, the best thing to do is to have a filtered faceplate on the master phone socket, and wire the router directly to the ADSL output on that, then run Cat5 back to the PC's, or use Homeplugs (which can be expensive). As long as you're using the Cat5 for ethernet and not as a phone cable, there will be no problems up to 100 metres (328 feet) between switches and routers, and no degradation in performance.

Thanks JoeKidd and Zebedeee. I will follow your advice, but will go for cat5 rather than homeplugs.

An update...

I today removed the i-plate and instead fitted an ADSL nation xte-2005, connected the router directly to that and have run a long external grade cat5e from here to a new Linksys switch and the pcs.

It was all pretty straight-forward and I learnt how to make my own cat5e, using a rj45 crimper!

The only snag was that one of the long screws snapped in half in the BT side of the NTE5 socket, so I have one screw and some heavy duty double sided tape on the other securing the ADSL nation faceplate to the backplate !!

The router took a lot longer than usual to connect and synced at 5000, compared to 3800 earlier, before I did all this. I then turned off the router and the damn thing has synced at 3900!!! I will leave it overnight and reboot early morning to see if this improves. I know that syncing at 4pm on a Saturday is probably not going to give the best result., so I am keeping my fingers crossed.

The best time to reboot is mid-day. Hopefully you will have enough spare noise margin to avoid a re-sync overnight.

Rather good news, but no sooner had I posted an update and my router is reporting a much higher sync:

DSL Status: Up
DSL Modulation Mode: GDMT
DSL Path Mode: FAST
Downstream Rate: 6432 Kbps
Upstream Rate: 448 Kbps
Downstream Margin: 12 db
Upstream Margin: 24 db
Downstream Line Attenuation: 36
Upstream Line Attenuation: 20
Downstream Transmit Power: 0
Upstream Transmit Power: 0

I guess I will need to wait a few days before BT up my profile from 2500!

Thanks to everyone for all the help and advice. This has been £71 well spent (on cable, crimper, krone tool, faceplate etc)