Phileas, I'm often inclined to agree with you but on this occasion you might be barking up the wrong tree a bit. As davidjohnson6 points out this has a maximum capacity of 1600 over an entire summer - once a week on a 46 seat ATR 42 not being what one might call a high volume operation! Take your point on timings, but the fact is it has worked before on a very similar basis. Similar flights to JER from a seemingly bewildering array of regional airports (like Gloucestershire as discussed but also Cambridge, Humberside and others) which are operated on an almost identical basis (seasonal, Saturday-only) except sometimes using somewhat larger aircraft (78 seat BE DH8Qs). And yet they come back, year-on-year.
Given that this is such a similar operation which has worked in the past, what's the problem?