KB:

Thanks for the considered and useful reply.

In the case of the previous conviction, it is usual in the UK that this would not be disclosed in court, which may help me. But there are things I said in the police interview that indicate clearly that I am an MAA.
The fact that alongside the 3 spam images, there were 68000 legal images which the CPS have already disregarded. Despuite this, I'm sure the police will have the right to introduce these into the proceedings.
I will ,of course, make every effort for this not to happen, and I will consult with my brief to try and achieve this.
I am due to appear at the Magistrates Court (lower court) for the initial hearing, in about 7-10 days time. At this point the police (I have been told) will attempt to have the court impound my new PC, together with any equipment in my possession capable of taking an image. I am making arrangements to ensure I do not own ;-) any such equipment on the date of the hearing.
The PC is a little more tricky. I will fight hard to be allowed to keep it, firstly, I am using the PC as a research tool for my defense, and to be deprived of it would harm my preparation. Secondly, I need my PC to do my job, and the loss of it would soon be discovered by my employer, which would quickly lead to dismissal.
I will be pleading not guilty, regardless of the consequences. I will be taking a moral stand, a stand that will allow me dignity. I am not going to roll over and play dead just for an easy life.
You're right about the UK law, they recently added the words "appears to be" before the words "under 18" to catch out the barely legal range of material.
I am considering hiring a defense forensic expert, although I'm not convinced that the expense will warrant what facts he can uncover. There are only 3 images to worry about, I know there origin and, in the case of the two "undeleted" ones, when they were downloaded.
My mitigation notes are becoming quite lengthy, but there are a lot of irrelevant sections that dealt with things other than the 3 images.
I will be posting an edited version of my mitigation notes here on GC and would welcome any comments.

Thanks again.