Eventually my DT400 is there, and so yesterday it came back to life (around the third kick too).
Keeping the revs below 4000 and just running up and down the lane, letting the new paint smoke, listening for any weird noises etc. I’ve fitted a clear pipe from the oil lube pump to the carb and noticed an occasional air bubble. The tank is full and I bled the pump, so all good there.
Is this normal?
I suppose with the standard black pipe, you’d never notice.

It may be normal but also might be the reason a black tube is uses so as to not loose sleep at night from worry. Same goes for fuel hoses but not under pump lifting pressure--if you call that pressure lifting it to the cylinder open hole. I wonder if its caused by the pump gears revolving & situation normal. By memory the pump bleed screw is only halfway up on the pump so not totally devoid of air. My other theory is the inlet pulses back with after the inlet sucking stroke so with piston on the way back down it blows back if the feed in is on the piston side of the reed valve but would get 'some' blowback regardless. Actually i guess your feed is into the carb small feed tube so stuffs that theory a bit. I'm keen to see what DEET says being the pump man. Not going to look at mine on my AT1 just incase haha.

Also some clear tube is larger ID than the tiny black stuff Yamaha uses.  Smaller line will not "drain" like larger tube will. 

Interesting thoughts, I have pre-mixed my fuel for the first gallon, just to be sure. I’ll probably just reduce the mix until I’m happy.

+1 to the two previous replies and the original pipe is still available from Yamaha, although it is probably sold in fixed lengths now and you cut to size.

Just to add on, I went the clear yellow-green tygon fantastalloy route and determined that it didn’t tolerate the cool Yamaha spring clips and wasn’t as though or as heat resistant as the original black Yamaha lines - probably because the original ones have lots of carbon black in formula - so I went back to original.  Same with the feed line from oil tank to pump. I think the Yamaha really engineered these bikes quite well which is one of the reasons i love them so much!

Think I might have found the cause, returned to the bike today to find this.