"When fixing a flat, you only raise the car until the tire is just barely off the ground."
I have always found that I need to raise the car a little higher than that, so that the tyre is at least one inch off the ground, otherwise it is a real PITA getting the wheel back on the studs. Just off the ground = easy to get the wheel off, difficult to get back on. When doing work under the car in my garage I often raise one or two wheels a good 6 or 8 inches off the ground (and then put axle stands underneath), and that's when I had problems with the pinch weld buckling before I started using pucks.

 

Thank you everyone for the responses (and for the lively banter too...) I'm concluding a few things from my initial question (and this is my personal interpretation)

1) The pinch weld clearly can take the weight of the car as the Jag jack uses that - but that's for temporary / emergency use, and is a lot of weight to put on a very small contact area. As 10 XF Premium says the angle could cause undue pressure and some people have had the pinch weld bend as a result - so this is one option but may cause bending...

2) A pinch weld adaptor / slot may help keep the jack straight or avoid it slipping, and protect the paintwork on the pinch weld. But ultimately the weld is still bearing the weight.

3) The diagram posted by Boiler clearly shows four jack points that are intended for use and do not have the same issues as the pinch weld. They offer large, flat surfaces to face a jack, and I will be using those as I see no need to risk trying the pinch weld as I have a large floor jack

One question - the rear two of those jacking points in the manual are very easy to find right in front of the back wheels and offer plenty of space to jack and to stand the car. The front ones as Jagular says are a long way in and I haven't quite worked out which piece of the car they are referring to - any tips on how to identify it as I don't want to punch a hole in the floorplate by mistake...

 

I looked and as far as I can see the inboard front lifting points are marked by longitudinal ridges protruding from the underfloor with what look like rubber strips on the ridges. I caution that I am not sure that's where they are but if not it's hard to think why those ridges are there.

The posted diagram is the same one I've seen elsewhere so seems a reliable document.

 

Check the back ones because on the S Type there is a plastic cover over the battery cable that runs from the trunk to the engine compartment.

There is room for a small jack pad but I have to remove the plastic cover and make sure the jack does not crush the battery cable.

I am unsure if the XF has this too but they are pretty much the same car underneath.
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Just changed the tires. The front jacking points are three sided, not just L shaped. If you use a floor jack on the pinch weld there is no way the front pinch weld can collapse, it is supported on both ends by steel webs.

I did find the right rear jacking point just inboard and behind the pinch weld rear jack point and used it. The left rear jacking point that should be in a similar location looks different. I wasn't sure exactly where on that point I should place the jack pad so I used the pinch weld as before.

The rear pinch weld jack points are not supported in the manner the front jack points are, presumably because they don't lift as much weight.

My floor jack is only a 1.5 ton aluminum low profile jack, not 2 ton.