Martin,  I've never seen that recipe before but the description that is posted in that tutorial does seem somewhat vague.  And it is odd, because some of those ingredients listed (boiled linseed oil, wallpaper paste, and bleach) I find lead to a softer PM when dried not harder product.  The hardest PM that I typically make is a result of 1 part paper pulp, about a half part saw dust, and about a half part calcium carbonate powder and of course white glue.
I find that boiled linseed oil makes the pulp more like cookie batter (bad) and less like bread dough (good), too much saw dust can make it brittle and rough, and bleach is unnecassary liquid, and commercial wallpaper paste dries too soft.
The trick to getting hard PM is getting the paper ground down to as fine a dust as possible and using calcium carbonate.
Of course, everybody on this forum has their own opinions on this subject so I'm sure you'll get lots of input (much of which contradicts what I have said!).  It's really just a matter of experimentation and fine-tuning what you like to work with as an artist.

The quantities I use (from an old theatrical props book, hence the imperial measures) which might help as a general guide:

10 oz paper - letter/photocopying quality, not newspaper because a lot of that's already been recycled and the fibres are weakened
1.75 oz wallpaper paste (dry) - I take sebrink's point about it being 'softer' but I find it makes the mix more model-able than just using pva. 
2 oz plaster of paris
2 oz polyfilla/tetrion filler mix
(or 4 oz polyfilla, but it's more expensive than p of p)
2 - 3 tablespoons white glue (pva).  (the original recipe used rabbits-foot glue, but this is a more acceptable - and less pongy - substitute)

Shred, soak and boil the paper as usual.  Blend it and strain as much water out as you can.  Mix in the wallpaper paste, followed by the p of p/filler - it seems to work better if you do this separately.   At this point I usually leave it for 15-20 minutes for the various fillers and pastes to take up the water.  Finally work in the white glue - I leave this till last because it is, of course, adding more liquid to the mix and if I've left too much water in at the pulp-making stage I can hold off on the pva so I don't get too 'wet' a mixture - takes much longer to dry and likely to distort while it's doing so.

This is a pretty tough mix, although you can play around with the quantities to suit yourself.   You can either add the  sawdust (in a couple of large handfuls for the above quantities) after the wallpaper paste which makes it stronger and gives a different surface finish, and add a bit more pva to compensate for the dry material. 

I agree with sebrink about linseed oil - I think that was a hangover from Victorian industrial production processes where the p-m was more or less baked - and not putting too much sawdust in.

Hope that's of help!