I think I've worked it out, lads.
I need to do more testing - but I think the runway needs to be at heading 000 for takeoffs to be successful - using either the DW Manual Method or the Orion Warrior P-3 manual method works.
Having the runway at a different heading seems to 'confuse' the plane - on the ground, the compass will start at 000 and then have to 'adjust' when you start the takeoff roll, causing very slow acceleration, a 'drift' to one side and then a crash.
I don't think bad weather or heavy loadout weight has any effect on takeoff (or landing) ability - but will still do more testing.
At least I'll leave this here for posterity in case other people have the same problem. Yay for this 19 year old game!
Cheers guys,
Stew