Jono its not fantasy land if you look at the national figures you will see that many can provide at 40% of the current NMW and do so.
£5.93 x 40 hours is £237 pw and 40% of this is £94.88.
Nationally the average HB in-payment figure for social housing is just over £79pw. Hence 40% of NMW is the current national social housing in-payment figure plus 20%. and is therefore both affordable and leaves plenty of scope