Red Ed

Why would a leak in an injector return pipe cause the engine to not start? I've seen this before so assumed this was my problem this time. I've replaced the banjo bolt tightening it carefully until I can see no more leaks anywhere but the engine won't start despite me turning it over and running a newly charged battery flat. I can see fuel in the fuel line and it's clearly reached the banjo bolt but there's not been a flicker of life from the engine.

My diesel engine wouldn't start and I discovered a leak in a hose (marked yellow, far right) which I replaced and all was well. About a year later car wouldn't start again. I suspected a leak again as could hear something hissing. I changed the remaining 3 lines (marked yellow) and noticed the nut (marked red) was loose. I tried to tighten it and sheared the head off the nut. Anyone know what this nut is and where I can buy another? Also can anyone share any good info / resources that explains all these lines as I have others which look very frayed and I wonder if they would benefit from replacement. Thanks in advance for any help, it seems there's fewer of these cars on the road these days which is a shame as I really love it.