Thanks everyone for the advice. Sorry it's taken me so long to respond.
The 3 hour feeding was the advice from both midwives and doctors at the hospital. It worked reasonably well while she was in special care, as they phoned me every time she needed feeding, while was always within 3 hours - whenever she woke up. However once we were at home, she was sleeping longer and waking her was taking ages and it was really difficult to get her to feed. Another midwife has told me that you should never wake a sleeping baby and that she'll let us know when she's hungry. This is working so much better for us - the night feeds are still roughly every 3 hours, sometimes longer, but they're on her terms and she seems much happier, although they are still taking a really long time.
I think part of the problem previously was that she wasn't latching on properly and I was really struggling was positions etc due to having had so much advice from different midwives & Drs after spending 2 weeks in hospital and feeling completely overwhelmed by it all. I've seen another midwife (the one who said not to wake her) who has really helped with the hold & the latch seems to be working (most of the time).
We're using a dummy occasionally now - just to calm her down. She's developed a slightly odd habit of chewing on her fingers, which I slightly concerned will become permanent and end her wrecking her teeth, so the dummy seems preferable - although it's not ideal.
I've started to express now - managed 20ml, 50ml, 20ml & 65ml in subsequent sessions - not sure if that is normal or on the low side. We started out using a cup to feed her but the midwife said that a bottle is better and she seems quite happy taking it that way. My plan is to express every day and do the 9pm feed by breast, then go to bed & let my husband do the midnight feed so I can get around 6 hours sleep before doing a 3am feed... Not sure that this will work but it's worth a try.
Although it's a lot better than it was, things are not perfect - feeds are still taking a really long time, which is ok in the day but really hard at night. Although I'm finding it easier, it's still really tough but I gather that's normal - I just wish that it was more acknowledged that breast feeding is really difficult. My NCT class led me to believe that it comes naturally to everyone.
I'm determined to keep at it until she's at 6 weeks & then reassess. I'd like to keep going for as long as possible but if it's still tough, I may have to stop or perhaps introduce a formula feed on occasion or daily.