The dog (six months, labrador) has a weeping eye and below the actual eye/ cornea, the eye 'covering' (is this the ciliary body?) is very red and covering now most of the eye. It's come on over about five hours and looks alarming. It might be starting in the other eye. I rang the local vet for an appointment, but they didn't have one that I can get to till Saturday morning. Should I push the point?
It sounds like your lab has some inflammation in his third eyelid. This can be caused by an allergy/allergic reaction or irritation to something that got into the eye. If there is swelling of that third eyelid, then it could possibly be what we call a 'cherry eye'. This again is just swelling of the gland that is present in the third eyelid that can happen due to allergic reaction.
Waiting until Saturday to be seen by your vet will not be a problem, especially since it does not seem to be bothering him. Some treatment options that might be presented are some antibiotic/anti-inflammatory eye drops/ointments to try and reduce the swelling, surgical replacement of the gland and third eyelid or removal of the gland. The treatment of choice depends on your veterinarians preferences.
For now, since this can be sometimes caused by allergies, you may want to try giving him some Benadryl and see if that might help with the inflammation/swelling. The dose of Benadryl is 1mg per pound of body weight every 4-6 hours as needed. Be sure to weigh your lab carefully and dose accordingly. This may make him a little sleepy so don't be alarmed if this happens. If the swelling doesn't seem to go down after 2-3 treatments then the Benadryl isn't working and you can stop using it.
I hope this answer helps. If you need any additional information, do not hesitate to ask!