It's possible that EE has put you on a banded profile
Looked at that and maybe your right. However we have had a development...
I was pretty convinced that it was not a wiring issue as the slow speeds were persisting. So being off work today I removed the Bright Box from the test socket this afternoon and replaced the faceplate filter. It failed to connect, with a (slow) flashing broadband light. I logged into the router and the stats showed it as synced with the exchange but not connected to the internet. I tried to phone technical support and got a message saying no one was available and they hung up on me

. After 10 mins the light settled to on, and I had internet connectivity. Stats...
Status
Configured Current
Line Status --- SHOWTIME
Link Type --- Fast Path
Operation Mode Automatic G992.5(ADSL2+)
Data Rate Information
Stream Type Actual Data Rate
Upstream 1055 (Kbps.)
Downstream 5167 (Kbps.)
Defect/Failure Indication
Operation Data Upstream Downstream
Noise Margin 6.7 dB 9.0 dB
Line Attenuation 25.9 dB 48.0 dB
Indicator Name Near End Indicator Far End Indicator
Output Power 12.8 dBm 0.0 dBm
Fast Path FEC Correction 0 0
Interleaved Path FEC Correction NA NA
Fast Path CRC Error 41 32
Interleaved Path CRC Error NA NA
Loss of Signal Defect 0 0
Fast Path HEC Error STR 121 58
Interleaved Path HEC Error NA NA
Error Seconds 34 24
Statistics
Received Cells 189331
Transmitted Cells 59996
Broadband Type: ADSL
Broadband State: Connected
Internet State: Connected
Time Connected: 00:06:50
Downstream Rate: 5167kbps
Upstream Rate: 1055kbps
BT Wholesale speed test, Download 4.61Mbps, Upload 0.85Mbps, Ping 41
Further Diagnostics...
Download 4.61Mbps (Maximum 4Mbps) (Acceptable range 2Mbps-7.15Mbps) Profile 4.56Mbps
Upload 0.85Mbps (Maximum 0.83Mbps) Profile 0.83Mbps
ThinkBroadband speed test, Download 4.4Mbps, Upload 0.91Mbps
The Bright Box was synced at 5167Kbps for around 10mins before it finally connected so the synced time is about 16mins, while connected time is only 6mins50secs. Still on ADSL2+ and fast path. Notable change is that the sync is slightly higher and the BT wholesale diagnostics states I can get up to 7.15Mbps with the acceptable range 2-7.15Mbps.
So I'm guessing I might experience a few resyncs upwards if the line has stabilised from what ever the problem was?