"Plus once you're in the 40% tax bracket, for every pound you earn over 44k, you only get 50pence of it, after tax and N.I."
I don't think that makes much difference, to be honest.
Right now, the marginal gain of earning £1 for DH is around 20p, because on his wage, the deductions of Tax, NI, housing benefit, CTC and WTC stacks up to at least 80p in the £. So an hour's overtime brings around £1.60 to the family income.
Although I think we forget what things were like in the 70s & 80s. My Dad worked for a fruit & veg distributor. Overtime rate was £1 per hour less than standard rate, which given the wage of the time was a huge proportion of his hourly wage.
People these days feel slighted if overtime is less than 1.5 times standard.