So i try and sign up for the unlimited 6 month free broadband offer, only to be told that i'm already using a promo code and thus i can't use another one so the free 6 month offer does not apply to me

Anyway i sign up as I haven't got broadband and paying one lump sum to Sky is easy and am promised speeds up to 15mb.
Router and letter arrive my activation date on the letter and MySky is 9th Feb, on the 9th I plug my router in and excellent i have broadband, look at my router stats they are stuck on 4mb - I read up about DLM and so leave my router switched on for 12 days, expecting to see my speed go up to around the 10- 15mb mark. I get nothing, no change!
I call Sky and one of their advisor says that there's been a problem and I wasn't activated on the 9th but rather the 19th

(so how come i could use the internet from the 9th?) Anyway he advises me of another 10 days DLM and my speed should increase at the end of it.
Check the stats a couple of times during this period and yes thats right still stuck on 4mb, today the 9th March a month after my initial activation date i check the stas again and still no improvement.
I call up again, get passed between 3 depts (Scotland, India and back to Scotland) Finally get through to someone who can test my line etc. She tests away (master socket, different filters, no other lines in, manual increases in speed etc) and finally says that 5.5mb is the max i can get on my line

from expecting 10mb to 15mb to get 5mb is very maddening. Now looking at my stats i believe i should be getting more. I have a brand spanking new phone line from the phone mast to my house, i only have the one phone point (which i've obviously filtered) she checked my line for noise and found nothing.
Is this all I can really get?
ADSL Link Downstream Upstream
Connection Speed 5208 kbps 509 kbps
Line Attenuation 34.0 db 16.8 db
Noise Margin 7.1 db 24.2 db
Is there anything I can do?