1995 Vauxhall Frontera engine management light comes on occassionally

hi there i bought my frontera last yr 98000 on the clock just had major service, full service history, ive had my exhaust changed from middle to back now have probs with heat shield when hot roars only advisory from mot is slight oil leak from rocker cover and exhaust mounts rusted or missing, ive never had a problem but a couple of months ago my engine management light came on only for a minute then nothing since until today a friend borrowed it to tow a horse trailer this is the first time its been used for this, and the management light was on the whole journey, when it was returned to me it was rested for 2 hours and when i drove it again my light has not come on, it was due service last month and havent done it yet has any1 got any ideas please many thanks lisa