If i recall correctly, a 2003 ecu should be passenger footwell under the carpet protected by a metal plate. Not 100% sure but there may be a warranty seal or something on it to show you if its been tampered with. Maybe a mapper could shed some light.

Besides, getting a mapper to have a look never hurt, they could tell if lambda reading or maf reading was out or whether it hadnt been tuned to said mods

well its in at a garage now getting a full diuagnostic check up....... the lad says it sounds like coil packs, but it'll be good to finally find out the problem!!