I heard that golden cheetah now has a similar funtion to trainerrroads virtual power.

Being a tight fisted git, does this mean I can make my turbo sessions a bit more interesting without resorting to paying a subscription to trainerroad?

I have mine running with a USB ANT+ stick giving live data overlay on Sufferfest videos, with target powers set from the matching erg files.
Setup help on the Golden Cheetah Google group. Erg db at http://73summits.com/ergdb/index.php

Alternatively look at PerfPro. One off cost to buy the software http://perfprostudio.com/ . Excellent piece of software.

Cheers, I've downloaded the software and had a play about, and ordered an ant+ dongle.

How easy is it to add a custom trainer, the GUI only has options for a few trainers and not mine.

All I really want to do is stuff like 2x20min FTP workouts as I use a singlespeed MTB for 90-120min group rides two or three times a week which is probably giving plenty of shorter/harder intervals even if it is a bit random and unstructured! I could just do it with a cheep computer and record speed, but if all it'll cost me is the dongle to get actual (estimated) power numbers then that's cool.

What trainer do you have?

minoura hypermag+ (but contemplating some sportcrafters progressive rollers from planet-x)

Minoura trainers are supported http://perfprostudio.com/StationaryTrainers.aspx . But maybe only limited trainers are offered in the 14-days trial version.

Sorry, i was still taking about GC. Intending not to spend much money on this as it's for such limited use. If it doesn't work I'll just record the rides on the edge and export to excel and apply a correction there.