Thanks for all the replies and my apologies for not getting back to everyone sooner!

Had a look at the glow plugs, 1,3,5 are testing < 2ohm, 2 and 4 are testing way over (30ohms and 3,000ohms iirc) so I'm guessing that's the problem, and will go ahead and replace all of them.

As far as the boost problem goes, the vacuum line from wastegate to outlet side of turbo is not interrupted, so doesn't appear to have an MBC installed. Quite a few cracked vacuum hoses around the engine bay, so I'm guessing that's not helping... Will clean the sensors mentioned and refresh some vacuum hoses and reevaluate.