Had a search but can't find it . There is a small hose from the expansion tank down to the metal hoses. Held with a clip at each end . I know it's a known weak spot and mine does have a large split in it . Not leaking yet surprisingly, although I am out later today so it's bound to . It's about 6 inches long total.
I have read it's not a part to buy individually, but others must have a solution. It may be different size at each end , I think I have read?

Any experience and solutions welcome, thank you.
2007 XK

 

A tighter clamp on the bottom with a hose that fits best you can find? Small diameter lines carry little actual pressure since such a small area.

 

Dealer says you can't buy the rubber hose alone, you have to buy the entire assembly of metal piping. Thats nutz - I just fabricated my own hose. Make sure that the spigot from the tank isn't the culprit - that's a weak point and the fluid runs downward, maybe looking like that the hose is the culprit.
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It's not actually leaking right now , but it has a split in in , see first pic.
I will experiment with some hoses over the next few days , perhaps one hose of the spigot size with a smaller hose pushed in for the lower connection. Made up with a metal sleeve for the join. Easy access but a short run for a fiddly hose .
It must be a common fail now , but there does not seem to be anything already made up,available here in the UK.