I cannot make out what Ms Riches & her co-objectors meant by this:-

'The morning-after pill was also, in their eyes, a special horror because it changed "the definition of the moment when human life starts from fertilisation to implantation"'

Can anyone explain, please, what they meant by this? In what sense do they use "implantation".

I think it is fairly clear, if lacking in precision. Let me give a clipping from a guide to pregnancy site:

After completing its six-day journey through your fallopian tube, the blastocyst you'll one day call your baby reaches its ultimate destination and begins to attach itself to the uterine lining. About 30 percent of the time, implantation bleeding will occur as that bundle of cells burrows its way into the uterine wall. Implantation bleeding, which is usually very scant and either light pink, light red, or light brown, occurs earlier than your expected period. Don't mistake it for your period, and don't worry about the bleeding — it's not a sign that something is wrong.

Now, if you want to have a clear definition of when life starts, as pro-lifers do, conception is nice and easy. However it is a simple fact that many fertilized ova do not implant in the womb for a variety of reasons, and we do not say these women were pregnant, if only because for most of human existence we simply had no way of noticing, (and even today it would be pushing the bounds of possibility).

So the next point you might consider is you wanted a definition is implantation. The 'morning-after' pill worked by preventing implantation. But if you can get the general population to think the best point to say life starts is at implantation rather than conception then Pro-Lifers have lost a vast amount of ground. Also since they typically believe life starts at conception, they regard the fertilized ova as killed exactly as much as by an abortion. Which is why the idea fills them with horror.

You don't have to agree with people to understand them!