herring for me Nick. see a lot of these in spring - that p10 will be brand new and casp have a much longer white tip as you know, head too blocky - usually eye is dark either cos not full adult but this only apparent when wings or tail open or if bird is an adult need to be much nearer to see pale in iris - though to my eyes your bird does have a hint of pale eye gling which I don't see in casp with genuine dark eyeif you are really wanting to sort it and have time there is a 2 part paper in an old BB by chris gibbins on casp ID. All my books etc are in storage as we move to Oxford on Friday. But loads of BB stuff is on net now and I think his paper is - I mention this cos your bird's bill looks wrong shape and length to me and he has a formula for measuring bill. I never got into it. For me the challenge for us inland Yorks birders is that a lot of our winter birds are L.a. argentus ie thosenorthern ones you'll be aware of. But even these have southern and northern populations - thing is they are often long legged and long billed with a big white P10 tip - if I remember Malling right he says their irises go dark temporarily at tutn of year, though I don't think that applies in March.I've been to Rufforth a fair bit and seen a number of these HGs and as well as all above they often don't have def pink legs (but then I've never seen a HG with a faint lime green wash to bill that genuine Casps often have).Anyway, I could be totally wrong on your bird in pic above - I find photos often lie!