p38 want start at all now

hi my p38 was running fine yesterday after 210 mile trip turned it off 10mins later went to start wouldnt start stright away i noticed the glow plug light was not coming on .so parked it up and went to it this morning glow plug light lit up and after a few turns it fired up went down the rd in it and it conked out managed to get it going again eventaly got it back home on the way it it held back a bit then flew all of a sudden parked it on the drive .now i have no light again .only when you crank the engine over the glow plug comes on faintly 'ive check all fuses can any body tell which relay is for the glow plugs and were is located plz thanks

210 miles ! - the poor thing is tired