Now the daffodils are out, along with celandines and a few early honest flowers
Although good to see, these plants have no bearing on your management nor on colony development. It used to be said that a guide to opening a hive was at the opening of the flowering currant, but that is another factoid of no use.
Look instead inside the hive on a windless day of 10-12C to check for BIAS, assess stores and laying space.
take the fondant out of the nuc or leave it in there?
As Ian said, only you can determine that. Base your decision on stores on board (say, a total of one frame for seven days) and the rate of incoming nectar (look in the combs).
presumably I now need to start to remove to make space for new frames?
You give insufficient information to enable guidance to be given at this distance. How many frames of brood are in the full colony, and what is the sum of frames of stored honey?
Lots of stores does not mean much, and as in the nuc, is nectar coming in?
A strong colony will need (roughly) the equivalent of two full frames, supposing it was unable to forage for seven days, at which point you would inspect and take action.
Do this soon, let us know the state of both colonies and we can then suggest a way forward.