So impressed with everyone on here. I am guessing that the failures and almost-failures like me aren't posting, but I will anyway, as it may encourage others to look for proper help before packing it in...
DD1 - 6 weeks, but she was tube-fed for 2 weeks of that and had a mixture of formula and EBM down her tube. Got her out of hospital feeding with nipple shields as this was the only way she'd latch on (refused to let a bottle near her, against all advice of nurses), but she wasn't gaining weight (c. 5.5 lbs at 6 week check) so I went cold turkey and switched to formula overnight. (It was hell!)
DD2 - stretched it out to 7 months, but mixed fed from about 3 months. Had terrible pain caused by bad latch, blisters on nipples the whole time... just agony, basically. But got some good advice just after introducing formula feeds a couple of times a day and the last couple of months were OK. But I went back to work at 7 months, and although I planned to feed her morning and evening she 'turned me down' at that stage.
My advice to anyone who's suffering and thinking of giving up (this isn't you, rickman, obviously, but possibly others reading this thread) is to keep looking for advice until you find what works. Wish I'd done it sooner... Had been advised by midwives, bf counsellors at the hospital, etc etc etc... but only got the advice I needed when it was already too late.