Td5 timing

Question : when the cylinder head is off and the top timing chain sprocket is sitting loose on the top chain guides, is it possible for the chain to slip on the bottom sprocket so that it may get one tooth out of sync?
Have set the timing up as per Haynes, with a lock on the flywheel through the bell housing then with the links aligned with the top sprocket mark and a further lock in place through the cam carrier to the rocker shaft, there is still a bit of come and go due to the oval holes of the 3 bolts sprocket to shaft. With the bolts at one extremity it runs very rough, but with the bolts at the other extremity it runs better but still not right. Feels like it ought to go back a bit more but no more adjustment available. Thoughts?

Hi no it cannot slip off just replaced a conrod on my 06 defender and that was a question I often wondered my self so whilst sump
and stiffener was off I checked and it won't drop off sprocket with these in place