"but do 60s English Diesel tractors have injectors"
ALL diesel engines have injectors of some sort.
Have you checked the oil? It's possible a seal leak has dumped diesel fuel into the crankcase, thinning the oil, and causing a rod bearing failure. (If that has happened, it will be way overfull on the dipstick and smell of diesel.)
While it's POSSIBLE you have a simple injector issue, that's probably not as likely to be causing a really SHARP knock vs. something more catastrophic. (More like a dull "thud".)
At this point I wouldn't run it anymore 'til you have a capable "wrench" with diesel experience check it out.
ONE thing you could do is to drain the oil into a non-metallic drain pan and swipe a magnetic pickup tool around in the drained oil and see if it picks up any debris.
If more than a simple injector problem, it's gonna cost as much as the tractor is worth to fix it, if you continue to run it 'til the engine "ventilates" the tractor will be reduced to scrap value.
All that being said, I own it's younger cousin, an IH "424".