If I am honest, I have never had a decent "clean break" the curds have always been very fragile and I have ended up with lumpy yoghurt to strain.
However, it has recently become impossible to obtain Vegeren in the local supermarkets, they just don't stock it. I have come across Langdale Essence of Rennet, which comes in a much larger bottle for a similar price, but is presumably more dilute as the junket recipe calls for 1 dessert spoon per pint of milk. I added 1 dessert spoon to 4 pints of milk and it had set solid by the time I checked it 30 minutes later. I cut the curds, left them and they separated nicely from the whey and drained in a fraction of the time they normally take. Hoorah!!
Dave