I'm sure this can't be right

[#1]

Plugged the router directly in the phone socket.
Now I have much faster slow broadband 
ADSL   
Type Interleave Path   
Status SHOWTIME   
                              Downstream Upstream
   
Data rate(Kbps)      352Kb  416Kb
Noise Margin                9          6
Output power(dBm)    119      137
Attenuation(dB)            83        31

   

[#2]

Quote

What kind of wire are you using for the extension?

Just a plain old telephone extension cable.
Used one at my last house wothout problems, but I was up at the 10Mb end of the spectrum so never really considered (or cared) that  it might be slowing my connection down.
Oh, the joys of country living.

[#3]

No-one has noticed the downstream power level.....
I would expect it to be much higher, say around 19, rather than 11.9. 
That's normally a symptom of wiring issues.

[#4]

Yes I think he realises now it could be down to his "Internal Wiring"...
@HP Sauce,
Have just sent a long PM about how to re-wire and given him some links to ebay supplier who is excellent on price and delivery.... so hopefully in a couple of days or so, we will get some good new from him !...

[#5]

It gets better 
Downstream Upstream
Data rate(Kbps)
448Kb 416Kb
Noise Margin
8 6
Output power(dBm)
119 142
Attenuation(dB)
82 31
Obviously my internal wiring was causing me problems.
Thanks to everyone that pointed me in the right direction.
@ Shutter
Thanks for the info re: ebay supplier. I'm currently running with an RJ45 cable (tucked under the skirting board & cunningly disguised by a potted plant) & a hub I scrounged from a friend.
I'll keep the info you sent me for a "rainny day" project - useful stuff.

[#6]

Glad to be of help.... 
(hmm .... It`s raining today ...........    )