I'm on Aol Silver. When the line comes into the house to a socket, I've got a doubler, into which, is a filter, from which the base of my home phone is connected. The lead from the doubler goes approx 8 metres upstairs to another socket I put in, to which I have another filter, the modem connects to this through a surge protector. Glad to say no problems. The instructions will tell you to put a filter wherever anything is connected to a line.
You definately need a filter at (c) -- the phone will then be plugged into the "phone" output of the filter, and your DSL modem will plug into the "DSL" output of the filter. Sounds like it will then need a DSL extension cable, instead of a regular telephone extension cable, to get to your PC.
I'm not familiar with a Sky setup, but I believe you'll also need a filter on your Sky outlet at (b) -- at that location nothing is plugged into the "DSL" output of the filter.
I spent hours trying to set up a broadband system for a lady and eventually resorted to ringing BT, they did a few tests, ( lucky I knew the engineer) and the problem was Sky. The lady never mentioned she had Sky or a line into it. I put a filter next to the Sky box and at last she was on line. AO Hell may be different, they tend to rule the roost once installed. Ruskle....