Peachsmuggler I was in exactly the same situation as you about 2 years ago.
Due to lots of helpful advice from Mumsnetters I agreed to go back 4 days a week but with slightly reduced hours 9.30 - 4.30 and a half hour lunch break.
I can't pretend it's not hard work, but then being at home 7 days a week is hard as well. The reduced hours may not sound like a lot, but it makes a big difference because we aren't rushing round like crazed loons in the morning, and in the evening DS isn't completely exhausted when I pick him up from the C/Ms. Incidentally DS loves his CM to bits, although clearly not as much as MummyDaddy ( we appear to be a united entity at the minute)
Ideally I would like 3 days a week, so I keep looking for potential job share partners or vacancies where that might be possible. Sadly with the financial climate the way it is and restructure after restructure, its hard to see how that would happen atm, but certainly if I had anotherDC I would be pushing hard for it. I do think you have more chance of getting down to 3 days a week in an existing position, rather than with a new company.
Also coincidentally enough the day after I returned to work, DH was made redundant. Luckily he found work soon enough, but being totally dependant on one salary is something to consider when you make your decision.
You say that 4 days a week wouldn't work for your family. Is there a particular reason for that ?
What I did is go back and made a conscious decision to review the position in 6 months time. If I hated it, then I would have given up, but I didn't so kept going.
How old is your DC ? Taking the maximum amount of extended Mat leave, and also any parental leave could make a difference to your decision.
Sorry feel like I am really pushing you towards WOHMdom and its not all plain sailing, if being SAHM is what you want to do then go for it, but do get a sense from your post that you feel like you are being pushed into a corner.