Sudden engine shake, followed by cut out and temporary non start 1.6 hdi

[MixerFistit]

While I'm on here for an unrelated fault, I'll revisit this while I remember.

The engine shake always came back when car was running hot. Not gentle hypermiling, but any spirited driving or 50/60 on hilly Welsh mountain A roads is enough to set it off. I noticed a slight wiff of fumes coinciding with the last occurrence and a little tick and surprise surprise, a little touch of black death around one of the injectors and it took a good few turns to tighten while hot. It was 3rd injector from left (every time I've had this on a car it's been that same injector).
The shaking hasn't come back but presumably damage is done so seal kit ordered along with the bolts as apparently once they start, they don't hold well.