my heart goes out to you both, bunglie and cheeseball.
bunglie, I too think will add my voice to the growing chorus saying 'send the letter to your son'
But I do understand your caution about content and timing, especially regarding your son telling your daughter.
I think you have to be very clear in your mind what this letter is meant to achieve. Is it mainly to tell your son the whole truth, give him the full details of your story - something in writing that he can study and refer to?
Or is the letters main purpose to get your son to meet you? to assure him that you love him but not give him too many details about the past, so it's more of an introduction and an invitation.
I know the letter could be both these things, but it might help you to be clear about the one major thing you want from it.
I think it's really important that you meet and talk to your son as soon as possible. You can then see his reactions, determine what you will tell him (or not tell him for now) as you go along, see if you need to pull back if he looks too overloaded with information and above all, show him all the love you have for him. It might need lots of meetings to tell him your story - others here have said better than I that it will take time. But I am sure your son must have seen the news about Sir Roy Meadows - have you thought about including any press cuttings with your letter? do you think that might help?
If you want my input, that letter is there to persuade your son to meet you first and foremost, so whatever you send, keep that as the goal.