Thanks for the comments everyone!
(glad to be back to the garden forum after a week away).
I did trim it regularly for cooking and drinking (say a handful a week) - and yes this kept it nice and bushy for 5 months, but when the yellowing started it just didn't seem to be enough anymore to encourage growth... and I didn't dare cut more/to the ground.
But the roots that I replanted are doing well now, fortunately. So if I ever have other problems I will be less concerned about cutting hard!
But of course I won't have any problems now
