Hi Jacqui, At 10 weeks from sowing these should be 30 – 40 cm high.
From your dates it seems they have been in these small pots for 7 weeks.
I would suspect two things. Overwatering and lack of nutrients.
Overwatering will cause the roots to die back. The plant is trying to stop itself drowning.
The overwatering will also wash out the nutrients from the compost faster, which in those pots would have run out in 3 – 4 weeks anyway.
I would suggest moving them on to slightly bigger pot, maybe 3cm bigger, with fresh compost.
I would also suggest planting deeper – up to where the first leaf comes off the stem. This will encourage fresh root growth from that area of stem currently exposed.
Try a different compost just in case what your using is suspect though if it were you would have had problems with other seedlings?
Water from the bottom to try and encourage the roots to spread downward.
Check the roots again in about 10 days when you should observe fresh root growth.
Don.