Any blue smoke from exhaust , especially on first start up ?
If you do have any then it could mean either valve stem oil seals are perished or valves not sealing in the head or piston rings worn.
If no blue smoke then it could be the clutch slipping as you mentioned "when using hill start assistant". A slipping clutch can sometimes smell like burnt oil if the friction material is contaminated.
The juddering could also point to a worn clutch.
Find a reutable local independant garage and ask them to investigate for you.
Hope it's not an engine issue as that can be very expensive to sort out.