Wow, so many responses!
I don't know how to interpret the materials information, so that is something new to learn.
Ok so now I know what saturation looks like on a 'scope - I don't think it's doing it, or if it is it's only slight. I'll have to use another source of square waves for the test jig since I don't have any schmidtt inverters. There is a bit of ringing though.
I have tried reducing it to the simplest form as a stand-alone converter, remaining with the dead-bug build. The input is at least 45 volts (and can go as high as 55) and the output is set to around 25 volts
Good point about caps on op-amp outputs, I'd forgotten all about that. However in the last incarnation before I started stripping it back to basics I did have a 150 ohm resistor in series there which would have stopped it being a problem. However, it was also ok before I put that in.
The chip, it's input and output caps, diode and feedback resistors are clustered as close as I could get them on a piece of plain copperclad FR2. The wire connecting the board to gnd is about 6" long, and it's input power lead is a little longer (I've now also threaded it through a really big ferrite bead). I'll try adding that extra ceramic cap on the input though.
The load is connected via mini croc leads, about 8" long I think, also via my meter so more leads there. The load's gnd lead currently goes back to the main cap and rectifier attached to the mains transformer. I'll try moving it to the copperclad and see what happens. Just wait whilst I do that... hold on...
Ok back again. That made no difference either.
I'll have to try building one on a proper pcb, though I've a feeling it won't be any different. There is a layout on the OnSemi version of the datasheet, so I'll base it on that.
For the time being I'm going to assume it's the cheap converter ICs that are at fault, but I'll have to live with them for now, since the real ones cost £4.43 including VAT. (this is the HV version don't forget) and ultimately I need 2. If it's still wonky I'll get branded inductors. The diode has stood quite a pounding so far, so I don't think I need to use a higher rated one. It hasn't measurably changed, but I suppose I should swap it for a new one just to be sure.
I'm not expecting to run this regularly at full output - chances are most of the time it will be loaded at less than 1A since it's part of a general purpose psu (still in development). If I get desperate I can always put in some kind of over-ride for the adjustment. Ugly method.
I'll try and take some photo's off the 'scope.