Peugeot 307 Oil Pressure warning message even though plenty of oil

Have the 'Stop oil pressure too low' warning light coming on intermittently. Local Peugeot garage said it was a seal problem and they fixed it (80-90% certainty they said). Took car back and same warning light came on after 6-7 miles or so. Plenty of oil in the engine. No sign of any leak after leaving in our garage overnight. Garage now says may be more serious problem -camshaft etc. No signs of fumes etc. in exhaust, engine running normally, no unusual noises.