Job done, was a doddle (more by luck than anything) but you have to measure and cut really carefully.
Firstly, here's the bits you need.
Here's a link to the realOEM page with the part numbers.
RealOEM.com * BMW E46 320i Partition trunk
You'll need, numbers 13 to 22, excluding number 17. I'm not sure what 17 is for except perhaps to cover the thread holes but it's not required here anyway so save yourself a few pence. Obviously you need 1 of each of these parts for each tether point you want to add. I only needed one and the parts in total came to about £40 so be prepared if you want to do 3. It may be worth going to a breaker first unless you are rolling in dough. It's also worth noting that for some reason my BMW dealer omitted to include the cover for the bolt (16). Annoying but it doesn't look too bad so I'll get the cover at some point.
You will also need a number 11 from here, RealOEM.com * BMW E46 320i Rear window shelf / sun blind electr.. It's worth thinking about buying 2 and I'll explain why later. Don't forget to order it in the correct colour or you'll look really stupid.
Now, on to the job. You will need the following,
A 13mm socket or spanner. A number 40 Torx driver. A very sharp blade. Pencil. Ruler. A 19mm spade drill bit. A sharp needle type thing (long thin screwdriver will do).
From the boot you can see the anchor point here,
Simply stick your sharp object through the threaded hole from the boot gently until it comes through the rear shelf so you know where to start cutting. Then using the small hole as a guide, cut a hole just big enough to get the bracket in. The shelf is only a few mm thick and cuts easily but take your time because if this goes wrong it's a new shelf at around £200. Now, look at the cover. You will have a round template already marked on it. This is for the bush at the front of the anchor point to poke through. You will need to measure very carefully with the bracket to line this up properly so the cover fits perfectly or it will look like a right mess.
You should now have a horrible hole that looks like this,
Note the insulation that you need to remove underneath to put the bracket on. It's already pre cut so just pull it out carefully.
Remember I said buy 2 covers? Here's why, you now have three choices.
You can either leave the cover off and fit the anchor point,
Then when you've finished with the anchor point simply remove it and fit the cover,
Or preferably, buy 2 then when the kids are older remove the anchor point and replace with a plain cover.
Anyway, I digress. Using the 19mm spade bit, drill the bush hole out of the cover. Then simply bolt the bracket to the boot then put the cover on, then bolt the tether point to the bracket and you're done.
It looks quite neat and when mini me is a bit older I'll just buy a new cover and remove the tether point. So there you go, for a modest sum and an hours careful work I now have a full ISOFIX top tether point in my E46 and a happy little man. Any questions please ask.
Next project is fitting iPhone and bluetooth integration with the stock stereo.