Grace really wasn't getting enough at the breast, after a marathon feed lasting from about 4pm to 10.30pm last night and then this morning me being able to pump 5.5oz after she'd spent 2.5hrs at the breast, I finally knew we had a problem, rather than just being worried that we might have a problem.
We met with a lactation consultant this afternoon, she listened to our story and watched Grace feed, we also weighed Grace before and after a feed to see how much she was getting at it turned out to be a tiny 6ml. Her theory is that the tongue tie isn't likely to be the issue and that it's just her not quite having figured out how to suck quite right, probably as a consequence of being born at 37 weeks. Her suggested plan is that I limit Grace's time at the breast to 10mins each side, rather than let her carry on indefinitely, then supplement her and pump, so that we're working on rebuilding my milk supply, getting her fed and trying to teach her to suck properly at the breast.
We also managed to get in to see Grace's doctor and went ahead with getting the tongue tie snipped, as it seems to be a fairly benign intervention and without doing it, we'd probably always be wondering if it would help.
We were advised to supplement either by bottle, or finger feeding, we're currently going with the finger feeding to try and avoid nipple confusion issues. Looks like I'll be spending a lot of time hooked to a breast pump over the next few days, but that could potentially be easier than her draining behaviour last night.