Well I eventually got round to doing this install.
I went for the Dietz 1491 DVB-T, 1451 IMU, with the 1477 (IMU>RNS-E) & 1478 (IMU>DVB-T) cables.
Got the lot from http://getitfast.covadis.de in Germany. Was about 150GBP cheaper than Dietz and 100GBP cheaper than some of the other German sites. Doesn't take cards so I had to do a bank transfer which cost about 20 quid I think.
I previously installed a Panasonic CX-DH801 8 disc DVD changer in the glovebox and was using this as a CD changer. The plan was to add this as an Aux in to the DVB-T and then have display on 2 rear screens and the RNS-E. I had already bought a video amp from eBay .
Starting at the front I tied in the CAN Hi and CAN Lo wires from the IMU behind the RNS-E. Not a lot of space behind the glovebox, but I left the 1451 IMU sitting there.
Took passenger seat out and then ran 2 AV cables (DVD>DVB-T & DVB-T >IMU) , power and DVB-T control cables from front to back. Also ran in the AV cables and power for the rear screens too.
Wired up all the power, ground etc... and plonked the DVB-T in the rear left side of luggage compartment. I wanted to put the video amp there too, but was limited by the length of AV cables I had, so that is currently sitting under rear seat..... not ideal , but it works !
Sort of put everything back together and tried it out. RNS-E picked up that TV was installed and gave that option under the CD/MP3/TV source select. This was without recoding with Vag-Com. I did later change Vag coding from 0301113 to 0301114 but it didn't seem to do anything, so I changed it back in case that was only for OEM TV tuner.
Picture is OK, just with the two active antenna randomly perched in back quarterlight window.
I did have to do a channel search a few miles away from my house to pick up the welsh transmitter channels, the ones I got initially were from english transmitter. I can now pick whichever one gives the best signal.
Functionality is pretty good, I like the EPG guide, it's much better than the one I had on my old set top box !
One minor mishap , I couldn't get change channel, menu or EPG guide to work from RNS-E buttons, even though they worked fine with the remote. Eventually tracked it down to a broken wire on the control cable from IMU to DVB-T. It looks like with everything behind the glovebox, it had got pulled too tight and stripped out of it's plug. Resoldered it on (easier said than done with wire about 3 copper atoms thick), and it now works fine. I also moved the IMU an inch to give a bit of relief on the cable.
Test drive showed up some audio interference so I swapped the permament 12V feed to the DVB-T for a switched ignition one as suggested in this post http://www.navplus.us/forum/viewtopic.php?t=3258&start=22. This made things quite a bit better, not totally gone but not noticeable when TV or DVD playing. I think I still have the 1451 IMU running off permanent 12V, so that's the next step, but that means glovebox off again for the millionth time....
All in all I'm pretty happy with the Dietz DVB-T. I haven't seen the Audi OEM one so can't compare, but this certainly meets my needs.
Next thing to do is to get VIM enabled, no room anywhere for 1280 so it's going to have to be an unlock job.