Thank you very much for your reply. I feel pretty stupid - I was very happy with CM and only wanted to try out the stock rom, never would've done so if I had realized it was not easy to go back.
Any ideas then about a possible resolution for the random reboots? Never happened on this tablet before, stock or CM. When I flashed this stock lollipop rom I did not do a full wipe, I did wipe the caches.
---------- Post added at 10:37 AM ---------- Previous post was at 10:00 AM ----------
Well I've gone from bad to worse

I used those instructions - I had used this process before to gain root - and got the the part in the procedure where flashtool reboots the device to flash 10.5.1.A.0.283.
Now the tablet cycles through the Sony Xperia screen over and over - it will not boot to system and will not go into flashmode. I can get into fastboot mode but I'm not sure what I can do to resolve this from there.
Again, any help is appreciated.
---------- Post added at 10:55 AM ---------- Previous post was at 10:37 AM ----------
OK - looks like I got it - I simply flashed the twrp recovery image in fastboot mode and that did the trick - restoring my lovely cm 12 now.
All set - thanks again for pointing me in the right direction.