Righty ho - I've tried the router in the test (hidden) socket of the master socket, both direct and through a filter, and both ADSL2+ and ADSL2 (forced). Stats are below, but are consistent with the results I get on the usual wiring. It's this consistency that makes me think the issue is with Orange/EE - line problems would, I think, give more variable results (as well as being apparent on the quiet line test), while the fact that I've seen connection speeds around 10Mbps show that the line is capable of more than I currently get.
The question is what to do next - I'm loathed to try and work my way through customer support to someone who knows what they're talking about - the last time I had to do it, the operator ended up telling me that they couldn't support my non-orange router! (Which wasn't the issue - they'd "regraded" my then upto 8meg connection back to 2meg

)
Anyway - the stats; Two connections each at ADSL2 and ADSL2+, both with and without a filter used.
Direct connection to master test socket:
DSL Status: Up
DSL Modulation Mode: ADSL2+
Number of failed trainings: 1
ISP DSLAM Chipset ATUC Code & Brand: -- Error getting the brand
DSP Datapump version: [7.04.03.00] Annex A
DSL Path Mode: FAST
Downstream Rate: 5165 Kbps
Upstream Rate: 1063 Kbps
Downstream Margin: 13 db
Upstream Margin: 6 db
Downstream Line Attenuation: 23
Upstream Line Attenuation: 13
Downstream Transmit Power: 21
Upstream Transmit Power: 10
Downstream Max Attain Bitrate: 5192 Kbps
Upstream Max Attain Bitrate: 1064000 Kbps
DSL Status: Up
DSL Modulation Mode: NOT TRAINED (ADSL2)
Number of failed trainings: 1
ISP DSLAM Chipset ATUC Code & Brand: IFTN -- Infineon
DSP Datapump version: [7.04.03.00] Annex A
DSL Path Mode: FAST
Downstream Rate: 6801 Kbps
Upstream Rate: 1091 Kbps
Downstream Margin: 15 db
Upstream Margin: 6 db
Downstream Line Attenuation: 23
Upstream Line Attenuation: 13
Downstream Transmit Power: 19
Upstream Transmit Power: 11
Downstream Max Attain Bitrate: 6828 Kbps
Upstream Max Attain Bitrate: 1092000 Kbps
DSL Status: Up
DSL Modulation Mode: NOT TRAINED
Number of failed trainings: 1
ISP DSLAM Chipset ATUC Code & Brand: IFTN -- Infineon
DSP Datapump version: [7.04.03.00] Annex A
DSL Path Mode: FAST
Downstream Rate: 6385 Kbps
Upstream Rate: 1079 Kbps
Downstream Margin: 13 db
Upstream Margin: 6 db
Downstream Line Attenuation: 23
Upstream Line Attenuation: 13
Downstream Transmit Power: 19
Upstream Transmit Power: 11
Downstream Max Attain Bitrate: 6412 Kbps
Upstream Max Attain Bitrate: 1080000 Kbps
DSL Status: Up
DSL Modulation Mode: ADSL2+
Number of failed trainings: 1
ISP DSLAM Chipset ATUC Code & Brand: IFTN -- Infineon
DSP Datapump version: [7.04.03.00] Annex A
DSL Path Mode: FAST
Downstream Rate: 6058 Kbps
Upstream Rate: 1075 Kbps
Downstream Margin: 13 db
Upstream Margin: 7 db
Downstream Line Attenuation: 23
Upstream Line Attenuation: 13
Downstream Transmit Power: 21
Upstream Transmit Power: 10
Downstream Max Attain Bitrate: 6084 Kbps
Upstream Max Attain Bitrate: 1076000 Kbps
Through adsl filter:
DSL Status: Up
DSL Modulation Mode: ADSL2+
Number of failed trainings: 1
ISP DSLAM Chipset ATUC Code & Brand: IFTN -- Infineon
DSP Datapump version: [7.04.03.00] Annex A
DSL Path Mode: FAST
Downstream Rate: 5057 Kbps
Upstream Rate: 1099 Kbps
Downstream Margin: 13 db
Upstream Margin: 6 db
Downstream Line Attenuation: 24
Upstream Line Attenuation: 13
Downstream Transmit Power: 21
Upstream Transmit Power: 11
Downstream Max Attain Bitrate: 5084 Kbps
Upstream Max Attain Bitrate: 1100000 Kbps
DSL Status: Up
DSL Modulation Mode: ADSL2+
Number of failed trainings: 1
ISP DSLAM Chipset ATUC Code & Brand: IFTN -- Infineon
DSP Datapump version: [7.04.03.00] Annex A
DSL Path Mode: FAST
Downstream Rate: 5070 Kbps
Upstream Rate: 1079 Kbps
Downstream Margin: 15 db
Upstream Margin: 7 db
Downstream Line Attenuation: 24
Upstream Line Attenuation: 13
Downstream Transmit Power: 21
Upstream Transmit Power: 11
Downstream Max Attain Bitrate: 5096 Kbps
Upstream Max Attain Bitrate: 1080000 Kbps
DSL Status: Up
DSL Modulation Mode: NOT TRAINED (ADSL2)
Number of failed trainings: 1
ISP DSLAM Chipset ATUC Code & Brand: IFTN -- Infineon
DSP Datapump version: [7.04.03.00] Annex A
DSL Path Mode: FAST
Downstream Rate: 6677 Kbps
Upstream Rate: 1079 Kbps
Downstream Margin: 13 db
Upstream Margin: 6 db
Downstream Line Attenuation: 23
Upstream Line Attenuation: 13
Downstream Transmit Power: 19
Upstream Transmit Power: 11
Downstream Max Attain Bitrate: 6704 Kbps
Upstream Max Attain Bitrate: 1080000 Kbps
DSL Status: Up
DSL Modulation Mode: NOT TRAINED
Number of failed trainings: 1
ISP DSLAM Chipset ATUC Code & Brand: ���� -- Error getting the brand
DSP Datapump version: [7.04.03.00] Annex A
DSL Path Mode: FAST
Downstream Rate: 6545 Kbps
Upstream Rate: 1087 Kbps
Downstream Margin: 12 db
Upstream Margin: 7 db
Downstream Line Attenuation: 23
Upstream Line Attenuation: 13
Downstream Transmit Power: 19
Upstream Transmit Power: 11
Downstream Max Attain Bitrate: 6572 Kbps
Upstream Max Attain Bitrate: 1088000 Kbps
Again, my thanks for your input