Happened across a copy of the music to this, and it doesn't look too hard in the grand scheme of things.

Is it Diploma level because of the interpretation/tone colour requirements - notes wise it looks no worse than a lot of the stuff at Grade VI/VII (apart from the key signature/accidentals all over the place)? Or are there some hidden pitfalls lurking around?

Syrnx has been used for many years as a Grade 6 or 8 set piece - the majority of it lies very easily under the fingers.
However, as with all standards it not really what you do, but HOW you do it that counts. From a grade 6 pupil I would be prepared to accept the minimum level of interpretation and control. From a diploma student I would expect a performance that delighted the ear: sustained my interest, used dynamics and colours to full effect, was perfect in intonation and showed a full understanding of style both historical and contextual.

Thought it was... Wouldn't want to sight read it (without a handy pen to scribble the more obvious names for some of the flats/double flats) but eminently playable with a little bit of work. Equally, perfecting the thing looks like it could take ages to be happy with the interpretation.

Faure Sicilienne seems fairly similar in type - deceptively simple, quite accessible, but lots of expression required!