Thanks Ronsimpson, the LTC3721 has slope compensation and a rather low current sense voltage reference, and takes longer to simulate.
The LT1243 is very simple and straightforward.
(I like the LTC3721 though and if i was doing it on the bench i would consider using it in some cases.)
The attached (LTspice and schem) shows how it can be done with the LT1243 …but I am getting an unfortunate little spike in the waveform streams as shown.