When making cider, can you carbonate it using tablets? and should this be in the demijohn or in the bottles?
Sure, you can use them and they'll work just fine. I've heard the carb levels can be low with 1 tab per bottle, so you'll have to experiment.
(As an aside, a little cheaper method that a lot of cider makers do is to thaw out a can of frozen apple juice concentrate and dump it into a 5 gal batch before bottling - or about 8-10oz of Apple Juice per gallon can be used as well.)
You can carbonate cider by bottle conditioning exactly the same way you would with beer. Either use tabs or boil some form of sugar in a small amount of water, cool and add it to the bucket prior to bottling. Make sure it is mixed well or the carbonation will be inconsistent between bottles. Also, don't bottle until fermentation is complete, or they will end up overcarbonated.
My last cider was carbonated with 4.5oz of corn sugar (the standard amount for carbonating beer). The level of carbonation seemed appropriate to me.