Post Office Broadband speed variations.

Zxytel AMG1302-T10. adsl2+. Op[enreach wall socket. I extension.
1.1km from exchange. 0.5km from cabinet.
Been with PO over a year. Had been getting download speed
13-14 Mbps until September when the speed fell to 9 Mbps - Att enuation 24dB, Margin 10dB.
Post Office then retrained the DSL. Speed then showed variations up to 11 Mbps eventually after a couple of weeks stabilising at just under 9 Mbps.
Test skt gives 10.5 Mbps.
Bell wires off gives 10.5 Mbps.
Extension phone was disconnected for these tests.
PO Tech tell me 11Mbps is the most I can expect.
Coolweb and Kitzx calculators shows I should get 17 Mbps - even with 24dB attenuation (which suggests 1.7km to exchange).
Can someone please enlighten me?!

Possibly the DLM that TalkTalk LLU is using has capped you

In reply to a post by MrSaffron:

Possibly the DLM that TalkTalk LLU is using has capped you

If it is then it should be possible for the op to request a change in profile, which would change SNR to a lower level 6 or 3db and or also switch off interleaving , It just a question of the P.O support staff having a clue ,in what dsl parameters are, and how to raise an order with TT to change things that could be the stumbling block for the OP