Hi Jack
just some statistics: Likelyhood of graft faliure is 3-6% in most studies (Bruckner and Kahn, CLinical Sports Medicine) within first 12 month. You will have put stress on the graft by overextending (normally allowed is extension to 0 degrees in the first two weeks) to some degree but it didn't sound as if there was a lot force behind it. I think those things made me think that most likely and hopefully you will not have damaged the graft. I think the best thing is to just get on with your rehab program and in due course to improve the hamstring strength and length on both sides (there are studies, that show that actually the other side underlies the same risk as the injured side!) together with the other aims of rehab (range of movement, strenght, proprioception, core strength and alignement)
Good luck with it