hello Guy,
I know the feeling with the back , got a duff one myself. As for rotavators/cultivators I haven't used a Mantis ,is that the little electric one ? ,anyway , I found that they are ok for turning already dug ground ie : just turning the ground over at the end of a year or in between crops. I bought a rotavator years ago to dig a plot ...it nearly crippled me ...lol . Mind you it was just after I had my big car smash ( in which I broke my neck and back ) , But I still found it easier to dig by hand first and then use the rotavator after. If you can, try borrowing one from someone or rent one ,that way you will see if you can get on with it ok first. My late dad had ( I think it was a Mantis) and he used it to till between plants and so on , it was fine for that . So , for digging fresh undug ground I would go at it with a spade and fork first . There after you could use a smaller tiller type thingy , but I find that they hurt my back more than just working by hand. The thing with rotavators is that they hit stones and the like and jump and kick over very violently and if you have a dodgy back it bl**dy hurts like hell , where as , if you are digging by hand you feel when you are doing just a bit too much, and can ease up or stop .
We are all different and what suits one won't suit another , so give it a go and see if you get on with them ...you may well do .... Good luck either way .
cheers
Russ