no alternator light

Hope someone can help. Just changed the coil and all was well 2 days ago. Tonight car will not start. Flat battery. Also the alternator light is not coming on. Did a jump start with spare battery and voltage across the battery terminals is only 6.8V!!!!! never seen that ever. is there a fuse controlling the charging circuit? if so can anyone tell me which one it is.

Car is 1990 1.8 clipper

For reference, the problem was that the blue wire at the alternator had broken. Well the copper wire had bit it was still attached by the plastic sheath. Took a bit of time working that one out. thanks to Rubjohhny for a previous post listing the blue wire as something to check.