OK, my understanding is this:
- the new wheel and tyre combination must NOT have a rolling diameter greater than 15mm, but obviously if you go from 16's to 20's then you're probably more likely to get problems;
- I think it's a TOTAL increase in track of 1" (or 26mm), so you can get wheels 1" wider on each side. If you've got the correct offset, it would mean that 0.5" would sit on each side of the centreline of each of the new wheels, so effectively that covers your TOTAL increase in track width. (I hope you can understand this - it makes sense in my head...)
- your wheels and tyres should NOT go out past the width of the gaurds, and the clearance between the wheel arch and wheels/tyres should be fine if you've chosen the correct width and offset to upgrade to. What you should worry more about is the height of the car off the ground AT ALL POINTS (chassis, crossmembers, suspension bits, exhaust, etc.). The minimum ground clearance is 100mm, any lower and you're looking at a defect.
ONCE AGAIN, DON'T TAKE THIS AS THE BIBLE, CHECK THE RTA WEBSITE AND READ IT FOR YOURSELF. Also, remember that some people (whether it's the RTA, the cops or whoever) may have a different interpretation of what the rules actually mean - it'll be your word against theirs if you have any problems...