Broadband dropping out - think it could be connected to Sky

[#1]

I have just helped my friend get connected online with Plusnet.  However having established a connection initially, it is now constantly dropping out and trying to reconnect.
She is a complete novice and so requires me to go over there to fix it if i can.
From my own experience i know that it's very easy to have an interruption on the line through dodgy extension cables and the like, however i didn't actually realise that her husbands sky box could be linked to the telephone socket, and am now wondering if this may be the problem.
She has three phones in operation, all these sockets i have put a microfilter in place.  However the socket in the living room, had a splitter in the socket with two cables coming out.  I unplugged the splitter, put a microfilter in the socket and then put the splitter into the microfilter.  Having googled this it seems i have done the right thing, however the connection keeps dropping.
I didn't actually look to see what cables were sitting in the splitter, i am assuming one phone and the sky box.  If this is the case should the configuration be any different?  Is it worth me buying a new splitter? 
I have put the router upstairs in one of the bedroom but i wonder whether i should attach it to the master socket?  Does the master socket look any different to the slave sockets or do you have to unscrew the front facing to see which is the master?
Thank you for any help.

[#2]

The sky box may be part of the problem...... can you disconnect it from the phone line...... for a short period, (an hour or so) and do a BT speed test......
The router (ideally) should be plugged into the master socket. i.e. the first socket where the telephone line comes into the house.... (May be in the front room . or by the front door..... depending on where the wire comes from the street).... You can see a pic of one here...
http://www.ispreview.co.uk/articles/week/socket.jpg
When you unscrew the two little screws, the plate comes away with a small plug that fits into the back of the socket on the wall.... Your other phones and services should all be "dead" when you have removed the plate... Plug into the telephone socket on the wall... (not the plate).... then you are eliminating the house wiring from the rest of the circuit
House wiring.... are the extensions hard wired? and fixed to the wall?.... they should be wired using this  
http://www.djsoundkit.co.uk/StoreImages/Large/E/E420B.jpg ; (black or white, doesn`t matter)
which is round with two sets of "twisted pair" single strand copper wire..... If you are using the DIY flat stuff, then it needs replacing. (except for telphone extensions from the extension socket)
You need to do an inspection inside the extension sockets and make sure that the wires are ONLY CONNECTED TO 2 & 5   (i.e. the middle pair) any other wire can be safely disconnected. ..... (DO NOT ATTEMPT TO CHANGE THE WIRING IN THE MASTER SOCKET )

[#3]

I can pretty much confirm that the Sky+ box is a contributor to BB dropping out. When mine started playing up I did the usual fault location process. I removed the sky box phone connection and borrowed it's ADSL filter to eliminate the one on my router as the cause. All worked fine and my router carried on as if nothing had happened once rebooted etc.
Sometime later, having reinstated my router back into the office I put a new ADSL filter in line to my Sky box and poof! Broadband dropped out immediately. I've removed the sky box from the phone line and rebooted the router and all is fine. I'm out of the contractual 12 month period so it will remain unplugged till the issue is resolved. At present awaiting the sky man to get back to me but he agrees it's probably the box at fault.