Thanks all! Much appreciated! And love all the food suggestions!
DD is a good "doer" generally (as we say in the horse world
) and eats quite a good range of food now (can't fail to be influenced by dh and my piggy influences and the constant cooking that goes on here/where we live - often three of the top news headlines are food or drink related !) but she feels full quite quickly so her portions are quite small. So you have to make whatever she eats count ifyswim as you only get one chance!
Cotswold despite eating well/having a good appetite, she has always been a string bean and on the lowest weight for her height percentile but we have managed (apart from a few dips) to keep her just on the right side of the line with a few noticeable dips. And, blow me, she is still growing! She is 170.5 cm now but only weighs 49 kilos. Which is a BMI of 17 which is too low apparently (should be 18.5 to 24.9). She looks like skin and bone today 
Goth I think drinking is the issue now you mention it. She is not good at taking in enough liquid at the best of times and if she doesn't drink, she doesn't feel hungry. And noodles - in chicken broth maybe - is an excellent idea.
Zoo sorry, should have explained properly, that is the problem. Neither of us are keen on dairy but dd is better than me with milk (I never touch the stuff). But she doesn't feel like drinking anything milky atm. She could be tempted by an ice cream/fruit based smoothy though- thanks!! I am intrigued by the nut butters too (not sure if they have arrived over here yet).
Sorry everyone. I know I am being overly obsessive/very PFB about this but it has been a constant worry. She is lacking in iron too despite supplements and eating shed loads of mince beef/steak with red peppers and broccoli.
It is still a shock to me that she is so thin as she was so very chunky as a baby. One of those baby's that people look at and say "gosh, she is looking very healthy" I couldn't see it at the time but looking back at the photos she was a right little beautiful porker 
Apologies! I'm just worried because she didn't have that much to lose in the first place and this bug has really set her back. Back to Flying ... !