After a bit of mess and noise cutting off the 20 scarfs with the router I have glued up the required length to make 10 stringers. It took me a while to figure out on the plan why 10 stringers are required as it would seem obvious that only 8 are needed. Well the remaining two are used as seat and bunk flat support along the planks.
The wood I used is carolina pine. My local supplier can only source 4.5 m lengths hence the scarfs. By the way I was particularly happy to buy (once more) wood from him: he's recently struggled after an arson attack damaged his stock and machines.