Just to say thank you to everybody :-)
I had to put the job off for a day waiting for decent safety goggles but I started at 10:30 today. The grout/cement is hard, it wasn't my imagination! I used a half moon diamond grout removal blade and it it finished the cut I had started manually very well, used the ends first (one seemed better than the other) then switched to the centre, when the dust colour changed from white to cement I knew I was deep enough. Did the other three sides then drilled several holes across about 1" from the top and used the diamond blade to turn it into a slot. I ensure I continued the slots to clear adjacent tiles.
One tap at the top on the sacrificial chisel (at least 40 years old) and the top 1" strip just fell out. Tapped the bottom part and the rest fell out in one! Finished at 10:47. The back is plaster, will scrape/sand it (don't want to switch from bruising my elbow to slicing it to ribbons) and then have a clean up.
Really pleased, thank you very much again for all the help!