Have built up running 12km on concrete in 6 mos but have gradual increasing pain in lower inner calf, what could be causing this?

Shin Splints. This is most likely what is known as shin splints, which is on the inner side of the shin bone where the calf muscles attach. Could also be a stress fracture, calf strain, or exertional compartment syndrome. If it doesn't get better with ice, stretching, and some cross training (stay off the concrete and swim or bike some), might want to see a specialist. Replace running shoes if over 6 months old.
Calf pain. It is still possible that you have developed a stress fracture in you tibia, but there are multiple other causes of this type of pain. You should consult with your doctor for evaluation and treatment.