I'm sorry, @Sophilicious, I went off to bed.
Can you tell me where you are in the country, roughly and I may have a recommendation for you? Call round local firms by all means but talk to a lawyer, not just the receptionist and see what you think about what they are telling you. If they clearly know what you are talking about, be guided by that, but if they just give you a vague 'oh yes, we do family law and I've heard of Parental Alienation,' run a mile. Not all lawyers accept that it exists and even fewer have experience of dealing with an alienating parent. It's the lawyer that's important not the firm, but the bigger firms have more lawyers with more experience. Unfortunately, the person I would have recommended retired last year.
The authorities, for the most part, don't accept it either. I'm not trying to frighten you, just help you get the best possible representation. It doesn't need to be someone on your doorstep if you're rural. Most lawyers will travel; you need someone city-based as that's where the experience is gained, if you see what I mean.
'PAMPA - Parental Alienation Member of Parliament Awareness' is a closed group on Facebook if you are on there. It's a closed group but ask to join and you'll get lots of help and support. It's primary function is to raise this issue in parliament and the commitment to join is to write a letter to your MP about this but there is a template letter to help you and masses and masses of information that you can read and use when you are trying to get the authorities/lawyers etc to understand what is happening to your children.
I'll pop back here again later today and if you can tell me where you are roughly, as I said I'll see if I can come up with a recommendation or two for you. Shop around until you are happy with the person you choose to represent you. Confidence in them is as important as their experience.
There is also 'Pathogenic Parenting Global Targeted Parents Support Group'. It's also a closed group and US based but it is a place to get support and recognition that you 're not alone in this situation and there are others if you look for them. Support is something that you may feel you need, or not, but it's there if you want it. Do talk to friends and family about things if you can and don't be afraid to ask for your help from your GP if you need it. Your mental health is as important as anything else and needs looking after.
Hang in there. This is a horrible situation to be in but all is not lost.