In December 2014 NATS began a General Aviation trial of ADS-B, allowing the linking of (certain) non-certified GPS systems to Mode S transponders. For detail see
NATS to enable ADS-B transponder functionality for GA | NATS
Details of how to join the trial were published in a blog
Helping General Aviation stand out from the crowd | NATS Blog
Being rather late to hear of this (I read about the trial in a GA fixed-wing publication over Christmas but only realised that I might actually take part in February when I get an email from a friend / examiner / forum member) I spoke with a CPL/FI who also flies the machine and our chief engineer and confirmed that we likely had the appropriate equipment. They opened communication with NATS, who were most helpful. NATS confirmed our equipment was suitable (GTX 330 Mode S and Garmin GNS 430), and even provided additional installation advice / material (which I think was from the equipment manufacturers).
There was some small expense - cabling, enabling the ADS-B (which for me was engineering time only, but depending on the software version installed you might have to update software), and a new aerial as the existing item was inadequate to handle the new data.
Engineering time was minimal.
System is now fully installed but not activated as we await authorisation from NATS to 'go live'.
Once the system is activated I will post observations / comments. In total there are seven pilots (including me) who fly the machine so I can gather views, rather than that being simply "The Thoughts of Chairman John".
Anyone else involved in the UK trial / care to post experience & views?