So for the benefit of PN and any other would-be trialists I am writing a few lines about the 80/20 product
The migration to the new speed was pretty effortless. The Time Capsule lost its PPPoE connection and simply reconnected within a minute or so. No issues with IP addresses (I'm on a static one) and I got upgraded from about 36/2 to 67/18 (or thereabouts), so a significant improvement but a little shy of the 80/20. This is obviously an inherent limitation of FTTC technology (oh, if only there were FTTP products out there!).
The service has been pretty consistent both in reliability and download speeds. I would imagine as more people migrate to the higher speeds more stress will be put on the ISPs infrastructure, but at the moment that is not noticeable at any time of the day or night.
The latencies for gaming have been consistently superb (I do have the Pro addon), so this has not changed from the days of 40/2.
With respect to the hardware, the Openzone Huawei modem seems quite capable of pumping out the data and the Time Capsule is, as expected, not missing a single beat even when I'm downloading at full pelt.
The one thing is that you do get a bit greedy. In a house of three geeks, (two of whom working in IT), the new speeds allow for the 120 GB monthly allowance to be drained at an alarming rate. So bear that in mind.
Whilst on that subject, it seems that the now-possible very large night-time usage breaks the "monthly usage" reports. If I download stuff all night long the system will tell me I've downloaded no data at all for the night.
All in all, then, a very positive experience from yours truly.
Thank you very much to PlusNet for allowing me to take part in this trial!
Modified at 14:00 to add [FTTC] prefix to the Subject header as requested.