According to navigeo.fr, customs upon prior request.
Infos douane : PREAVIS 2H00 PENDANT HOR AFIS OU DERNIER JOUR OUVRABLE AVT 1500UTC TEL: 33 (0)2 35 84 14 40
and that is confirmed in the "Carte VAC":
9 - Douanes, Police / Customs, Police : HOR AFIS : PN 2 HR 02 35 84 14 40
En dehors HOR AFIS : PN 24 HR ou dernier jour ouvrable avant 1500. Outside HOR AFIS : PN 24 HR or last opening day before 1500.
"opening day" better translates as "working day", I reckon.
Be warned that officially Dieppe R/T is in French only - but the same applies for LFAT Le Touquet and LFAC Calais, and I heard quite good English from both towers recently.
As for the flight plan: I hope you are aware there exists no such thing as a multiple-leg-flightplan. If your flight consists of three legs, then you'll have to file three flight plans; except for those legs that do not cross FIR boundaries, and are entirely outside controlled airspace.