I believe my disconnection issues were primarily due to an extended DSL cable, as I discovered similar issues with the Thomson in place.
I was moving the Draytek closer to the telephone master socket as part of Be's troubleshooting process when the power failed.
I would need to check on my firmware version as I'm at work at the moment...

Edit: Firmware is as follows:
Model Name : Vigor2850Vn
Firmware Version : 3.6.2_2471201
Build Date/Time : May 4 2012 11:29:50

Current firmware is as follows:
Model Name : Vigor2850Vn
Firmware Version : 3.6.2_2471201
Build Date/Time : May 4 2012 11:29:50

Note: I am not stating that this is the best firmware to use or not. This is the firmware that I'm using and I had no disconnect/reconnect issues for the first 6 weeks of using it.

Update:
I had BT here today. After some initial investigation, BT agreed that there was significant line noise. I asked the technician to change to a new phone socket as I noticed that connecting via the master socket versus the test socket made a noticeable difference to line speeds. The socket was replaced (back plate and face plate) and this pretty much removed all the noise. This left the technician scratching his head a little but he put it down to a possible lightning strike which had adversely affected an older phone socket type.

As a result of the replacement, I have fewer sync issues and a significantly higher sync speed. Where previously the sync speed would settle at about 5Mbps downstream (3.5Mbps on Speedtest.net), I now achieve a fairly constant 11.5Mbps downstream (9.97Mbps on Speedtest). :mrgreen:
I'm pretty happy with the improvements in the line quality. I'll monitor for a few days with the Thomson TG585v7 in place until I'm happy with the stability and then try the Draytek Vigor 2850vn again.

I'm not sure anyone is actually following this, but I will post again shortly with updates.

That's a great result with the line speed improvements!

Just remember that if you experience sync issues when you use the Draytek speak with BE and try that SP20 line profile. It did the trick for me and it doesn't take long to get it switched over either.

@mrjolly
Thanks for the advice.
I'll keep that in mind, once I've got the Draytek up and running.
(Connection up for 18 hours with a persistent line sync speed of 11.5Mbps.)
I don't normally do this, but, WOOHOO!

Always nice when things 'just work'. Be prepared that you might lose a big of speed with the draytek but hopefully be better to run your connection through just one device.