It's something that I've been struggling to understand myself over the last fortnight as I've been working with this recipe. I'm afraid I still don't understand it, despite working in physics.
Bliss's book is very UK-focused. It's a slim paperback with a good treatise on the science behind cheesemaking and then a few English cheese recipes, mostly focused on the cheddared and milled forms of cheese. Her Cheshire recipe calls for annato so I suspect it's for a red cheshire.
Apologies for my lazy typing, I did indeed mean Newton/square metre. To confuse things, Bliss refers to 24.9N/m2 as 5 cwt and 49.6 N/m2 as 10 cwt.
Trying to find conversions for those, I turned up [http://cheeseforum.org/forum/index.php/topic,10990.msg83813.html#msg83813][/this article] by JeffHamm, which suggests that while 1 cwt = 112 lb, 5 cwt = 7.13 psi and 10 cwt 14.23 psi. However Jeff's spreadsheet suggests that I would need 100kg of weight to achieve that...so somewhere there's an order of two magnitudes difference. 0.006 psi is about a 100g weight!
At the end of the day I opted to use the middle road and use 10kg and 20kg of weights as they seemed a normal range for pressing cheese.