If you were getting 8-10Mbps on ADSL (normal) broadband why did you go to the extra expense of VDSL (Infinity) to get an estimated 7-13Mbps? Even if you got the maximum of 13Mbps it would not in my opinion be worth the expense and as it now appears, the problem of changing for 3Mbps.
Hi, I was working on the proviso that as I was supposed to be getter no a max of 7mb on copper but was achieving 8-10 the same might work for the upgrade. Oh well live and learn. I don't have that master socket I have this one.
You have essentially the same socket @imjolly showed (at least the back of it) with an Openreach Mk3 SSFP (Service Specific Face Plate) on it to which is connected your VDSL lead. If you unscrew the 2 screws the bottom panel with the phone lead (and any extensions) will come off. You can then remove the Mk3 SSFP which plugs into the NTE5a revealing the test socket inside.
The image on the right is like @imjolly 's. The Mk3 SSFP plugs into the test socket then the phone socket (with any extensions) is screwed back on. That's why your socket looks "3 deep". Back Box, NTE5a then SSFP. The guys need you to connect your modem to the test socket with a microfilter and post the results to see if they're any different.