Bad. Earth?
 

Hope someone can advise on an issue with a mates P1

ABS light flashed a few times, car missed a beat, Defi's went off, pulled up at the side of the road, every time the cooling fans kicked in Defi's reset.

Set off again, drove approx 100 miles since, no problems, any ideas please?

Sounds like it.
Check tightness of battery earth, and condition and tightness of all others, esp engine to battery.

Be an idea to disconnect and clean the earth cables as any corrosion will increase the resistance so reduce the current flow. Once cleaned give them a smear of copper based grease to help prevent any future build up of corrosion.

Thanks gents, done the above, trouble is we don't know if we've fixed it as only happened once, much appreciated

Do you have access to a multimeter as you could then test the resistance from various parts of the engine to the chassis then to the battery terminal. Firstly do a check from the actual terminal to the clamp and make a note of this. Not sure on your knowledge of electrics so with out being cheeky, the lower the resistance the higher the potential current flow. For a given wattage this means a lower voltage as current times volts equals power in watts.

Must confess never thought of that, not the best at electrics but do have a good general understanding, what you say makes perfect sense :) many thanks