Tannoyman,
Those glow plug heaters won,t last long -- guessing, about 60 seconds or less and diesel engines usually have timers on them -- when those timers fail they usually burn up the glow plugs !!
Normal on, is about 10 seconds !!
You of course, can test this yourself,and may get a little longer than I mentioned, but they won,t last long !!
Northern tool has a 12 volt portable heater and look on Ebay, you can find 12 volt emersion heaters for around $28...
I wish those glow plugs were more durable, I wind up replaceing a couple every year in my Ford diesel, and now keep a few in the glove box !!
Good luck and happy building, Bill H.....