The bottom ("primary") chain is a whole world of extra pain to do. As has been said, the sump needs to come off, but to get it off you need to take the exhaust off the bottom of the cat, and the centre bar from the subframe. Even then you need to take the extra casting off, which goes between the pressed steel sump pan and the bottom of the block. That, in turn, needs the aircon pump mounting bracket to come off. It's very difficult in situ. The whole engine is like that - stupid things which, if a fastener had been moved 10mm or so, wouldn't need to have been taken off...
When I did mine, I could find absolutely nothing at all wrong with the bottom chain. It was just a huge amount of extra work for nothing. I spoke several times to Des Marples at West Yorkshire Engine Services, who was extremely helpful when I was doing it. He confirmed that he's never seen the Primary chain fail. If you DO want to change it, there's a massive Torx bolt through one of the sprockets (I think on the diesel high pressure pump) I've a vague feeling it's a T70, so you'll need one of those (and a big bar....)
The other thing I learned the hard way, was not to change the cam sprockets. Mine weren't worn, but as I got a new pair in the kit, I thought I'd swap them. Big mistake! The bolts through the middle of each one are supposed to be torqued to something daft like 120 lbft, but the only thing you can hold the cam with, is a cast hexagon on the camshaft under one of the injectors (they could have put it anywhere on the length of the cam, but no, they had to put it in a place where you have to take an injector out....). Being "as cast" (or forged) it's a rubbish fit in any known open-ended spanner and you just end up rounding the flats off it trying to hold it with an open-ended spanner against the (bigger) bolt through the sprocket (which of course, you can get a socket on).
If you search for a thread called "timing chain misery", there are a number of photos on there, which might help. The only thing I WOULD say, is that about 15,000 miles after I did this, the car developed cold starting problems which could only be solved by moving the cam timing sensor slightly. This suggests that there was some sort of mismatch between the cam and the crank sensors, indicating that something was going out of tolerance with the valve timing, so maybe it DOES pay to either change everything (or possibly I'd done something else wrong, or bought some dodgy pattern parts....).