Got the truck back, now working nicely again.
The problem:
A few weeks back I changed the oil on the truck. Shortly after I was getting a smell of burning oil - I assumed this was because I had overfilled it slightly. A few days ago the burning smell stopped.
And the connection....
Was nothing to do with oil - seems the wiring on the preheater was substandard and was melting, thus giving off burning smell, which I linked to oil cos that what it smelt like.
All now rewired and moved away from block.
Fingers crossed all will be fine, hot water is ok now, but no fun when its -20 next week!
Pete