What nobbled my tomato plant?

Been growing toms for 30 years but this is the first time I've had one die due to being nobbled from the base. If I'd spotted it earlier I might have been able to save the plant, but we were way from home for three weeks and our live-in garden care assistant didn't spot the start of things.
Cause of death seems to be severe stem rot starting a couple of inches above soil level. The plant is the only one affected of 9 in our polytunnel.
My guess is that something nibbled the stem and that it then developed a fungal infection. But...what would nibble a stem a couple of inches above soil level?

 

That's a strange one Dave. The black things in the second picture from the bottom look as though they could be larvae, which suggests some sort of beetle or weevil. If it was any other form of pest I don't think it would have gone for the stem just in that area. Did the soil around the base of the plant look clean?

 

Soil is clean Sheal. I think the black blobs are fungal grows -0ught to have done an autopsy but too late now!
My guess is that something had a good nibble and then the rot set in.