Good suggestions fellas, thanks for the input!! :)

In the light of day it looks like one of the wires has been rubbing on the metal bracket which the fuse box is mounted on. The loom was packed in tight with no protection and i believe it has worn through then earthed on the metal etc etc. Only affected about 2" of two cables so intending to cut...

Unfortunate first post, Mrs rang earlier in a flap, she had started the car, 53 plate TD4 SE, all started fine then the horn came on permanently and she could smell burning. Panicked, turned it off and i was just round the corner so arrived shortly after. No smoke on arrival but visible burning...