Thanks for your advice, it's quite hard to find reviews or opinions on the Diplomat FP's. I've read that Diplomat's medium nibs write rather fine for a medium nib, and I do like fine writing. But I prefer smoothness over fineness.
By any chance do you know how the M nib holds up against the F nib?
No, sorry. In my experience, the M was on the finer side of medium, and is likely to be a touch smoother than the F, as wider nibs often are.