6.2 N/A wont start

I have a early 90's 6.2 non turbo in a 23ft Bayliner and whilst coming back from a fishing trip last week the engine started to loose power and finally died
Changed filters, bled lines (whilst bobbing around at anchor) but still refused to start, so got back in on the aux and went down today to try and sort it out
To cut a very long story short, after trying various things, we found that the return line fitting on the injector pump was blocked with what looked like metal
The engine would run fine when the cover off the top of the IP was removed, but when it was replaced it would die after a few seconds
All seems to be good now but should that fitting be completely clear, or should there be some sort of non return valve in there which I have now pushed out
Thanks in advance

There should be a check valve (non-return valve) at or after the return outlet at the IP. If removing it allows a start, it has failed closed (they almost always fail open). If it only starts with the IP cover off, the return line is blocked in some manner. The return line should be free-flow from the IP to the fuel tank, in any case, so verify that. It could be blocked anywhere between the IP and fuel tank (pinched/failed line, excessive tank pressure, etc.). The check valve only prevents drain-down (air allowed in through the return line) when the engine is not running. By "metal", what do you mean? Is it just a piece of metal (a normal part of the check valve), or shavings that may indicate the failure of another component? If shavings, locate and repair the source before it contaminates and/or destroys the entire fuel supply/delivery systems.

It looked metallic, but when I poked a thin screwdriver through it, just crud (grit etc) came out
I'm hoping the check valve is located within the IP and this threaded hose barb (for want of a better word) screws into it
No blockage on the return lines, just in this fitting

The check valve should be part of the outlet fitting. Being a marine application, who knows. It should have one, in any case, especially on a platform that may see infrequent running periods.

Bit of a late update, apparently the check valve fitting had a glass ball to act as a non return valve and this was set to crack (open) at around 6 psi
I found this out on google etc
I have fitted a new check valve that came from across the pond and the engine ran for a bit, then suffered the same fate and stopped
I had to take off the check valve and break the glass ball with a thin screwdriver to allow me to start the boat and get home
The boat runs OK without this check valve, but not 100% (at low speeds the revs start to climb on their own and I have to shut the throttle to decrease them)
What sort of damage has been done to the IP and will I cause more by running it without the check valve as it's obviously there for a reason

It sounds more like a problem with fuel supply to the injection pump, or a failed transfer pump inside the IP. It could also be the lift pump has failed or is too weak to keep up. Check fuel pressure at idle and under load (if you can). It should maintain steady 4 PSI at idle, and always above zero under the greatest load.