adjust your clutch mate i had that problem aswith getting into normal gears properly, but for now say if your parked up and you need to reverse put it in reverse (before switching engine on) making sure you keep your foot on the clutch pedal lol, then start up, and it wont make a grinding noise, but as said before get it adjsuted first, once adjusted whill feel a bit weired but you'll get used to it!
Ye it will be your clutch cable slipping mate. Just wind the nut up the bolt a little bit and put the retaining clip back on. It might be slipping because the retaining clip fell off mate in which case you need to buy one for 57p from Vauxhall. I had this problem on my car with the forward gears.
if its happened on every car youve driven then it doesnt sound like your pushing the clutch down far enough or are ramming it into reverse gear....doesnt matter whether its synchronised or not, every gear change should be smooth and quiet.
either that or you've had bad luck with choice of cars for clutch cables or clutch wear lol