Okay all, thanks for the helpful info so far. Here's is an update.
I checked under the car again and gave everything a clean. For everyone concerned about the cracked subframe. As I stated in a previous comment it is not cracked. Here in the UK the cars are undersealed from factory. This underseal cracks over time and water, grit and salt get in underneath. Hence the surface rust. I brushed away all the loose underseal and sprayed the area with rust converter, primer and finally some black paint.
I cleaned up the wet area and went back to find a clearish oily feeling liquid with little to no smell. If anything it smelt a little oily but thats it. No diesel.
Once again, the brake fluid in the reservoir isn't and never has budged. Still confused haha. Will get the mechanics to check next time it's in.
I was hoping someone would see this and immediately know the answer but clearly I was wrong! Any way thanks to all who have been constructive so far! It's much appreciated.