Well I gave it a go and my test today revealed that I have held my own over this terrible winter and my FTP remains at or about 305W, which is just fine. The test didn't go well, I managed to hold 356W for the 5 minute TT at the start of the test but I knew I still had some fatigue in my legs from Sunday's ErgVideo motorpacing fun and games. I knew early in the 20 minute TT, at 329W, that I wasn't going to get to the end and there was just no point in killing myself at this, or any other, point in my training.
So, my conclusion from the test was that 305W is a very realistic FTP upon which to base the next phase of my training, the spring build. I didn't waste the session and having had a go at the test I did 60 minutes of L2 and lower L3 so the total TSS for the training session was between 90 and 100, a good piece of work which will stand me in good stead.
Curently giving serious consideration to basing the majority of my spring training on ErgVideo workouts which would be quite a leap of faith, further investigation required on that one.