When you say 'disposable income' do you mean after all the bills, food, baby stuff etc is paid for? OH and I have just worked out our finances and I wrote a list of the things I like to buy but don't need, like clothes for me, coffee out, travel, birthdays and Christmas presents etc. We worked out that £150 a month should cover that, obviously not including splurges on clothes or TVs etc! I've only just stopped getting maternity allowance so I don't know how it will work out but £150 a month works in theory for me. The major expense was actually presents, I'm going to have to be careful to save each month to cover them.
ETA - I also do the food shopping, clothes for LO etc. Without cutting down on our lifestyle, £350 a month plus Child Benefit is more than adequate. I don't know about the other bills though, OH sorts that out.