Beetroot in raised bed (Trug)

I got a 6' x 3' true for Christmas and have planted beetroot, carrots, spring onions, lettuce and parsley all from seed.  Although very late coming through, they are all now thinned out and thriving except the beetroot (Bolthardy).  The leaves are withering and dying off although the stems and roots are still ok.  Can't see any peskies on the leaves at all.  Any ideas - I filled the trig with a mix of John Innes and compost.

Thanks

Meg