Its nothing to do with FON, you should be connecting at about 7904kbs, but something is restricting your bandwidth.
Can you please connect the home hub to the test socket as shown below, this will eliminate any internal wiring issues.
Do not worry about your "up" time, as that is not the reason for your slow speed. You only have a 6.7dB margin, so the speed is not going to increase, no matter how long you leave it, as something is restricting the available bandwidth.
Two things to check.
And is there any noise on your phone line?
Please can you dial 1470 17070 and select
option 2 (Quiet Line Test), and see if you can hear any loud crackling or
hissing noise on your line.
Did you check for any connections before the master socket, as described in the diagram?
Does your phone line come directly into the master socket from outside?
The ADSL splitter is fine, provided that the home hub plugs directly into the front.