So I’ve just taken a few weeks off training and built a new pain cave and also bought a power meter. My fitness has slid by a fair amount but it appears my smart trainer is also off by around 20w compared to the PM. I Intend to use my power meter both indoor and out going forwards but my fitness signature from previous data is now incorrect. Do I need to do a reset of some sort and start again?
What type of trainer did you get? I find my Tacx Flux 2 reads ~10-12% lower than my power meter. It’s a pedal-based power meter so I just switch it between bikes and use it everywhere for consistency.
You could use the Power Curve & Calculator to adjust your signature, if you are confident that you have three reasonably accurate values, or in-/decrease your old values with 20 Watts and then recalculate.
Then again, doing your workouts with your new PM both indoors and out, Xert will quickly give you new values, with or without a BT.
Aiming for a BT is preferable though, but if your trainer is reporting 20 Watts more than your new PM, that’s gonna be hard…
Your old data is fine, since the FS was scaled to that power source.
A maximal effort/breakthrough with the new device would be a good start. If you’re not quite able to breakthrough, then it’s possible to manually lower the F.S. to adjust to the new power source (and it will go back up with training).
Thanks for the advice folks, did keep taking your turn this morning, tried it at the old FS, managed 2 sets, dropped it 10% and managed a couple more but then had to alter my FTP on zwift to complete 
So looks like my FS - 30w is about right, just need to suck it up, do the hard work and get the numbers back up, but genuine numbers this time rather than false flattery!