If you have a backup of your user sounds I think you can turn it off
and try to restart it after 10 seconds or so ..... 
Cheers !
Thanks, unfortunately I don't have backup so I'll wait a little longer as it's still stuck at the same place. So that's the only risk of restarting even if the cycles not done? But does it seem normal with such a long tuning time with extreme temperature changes?
I've had my P5 get stuck twice during tuning. I wrote Sequential support and they suggested I clear the calibration tables. I did so and haven't had an issues with tuning since - neither the synth getting stuck during the process or my P5 needing frequent tuning. The heatwave is probably doing quite a number on those calibration tables.
Andy at Seq support also suggested I write down the readout next time the synth gets stuck during tuning, but I haven't had any problems since, and thus ain't written anything down. I do know that my 5 got stuck at two different places during its run of tuning freezes, but I can't remember what was displayed.
I'd suggest sending your info to Sequential. Sounds like a known issue and the more info they get from us, the better for all.