The 4 bolts that go into the bottom of the front lower ball joint...:
Ball Joint Special Bolt - CBC1805 | Jaguar X300 - XJR, Jaguar XJ40, Jaguar XJS | Jaguar | British Parts UK

Does anyone know what size/thread they are?

Yes, 2 of them sheared off on me... & that's after having to get the grinder out for the bolts on the top balljoint too!

 

Nobody? It's close to M8x1.0mm but I don't think that's quite right.
I'm guessing it's some imperial size? My rough measurement makes it 9/32 x24 but that's quite obscure...

I've ordered the replacement bolts, just need a tap!

 

Actually, I reckon it's 5/16 x 24tpi, so that's rather more common.