OMG, what a saga this has turned into!!!
mister-auto replied to my second message and said they checked with their suppliers and they don't know when it will be available in stock, so they cancelled the order rather than put it on back order - fair enough really, better than just leaving me hanging.
More hunting round has lead me to think there might be an equivalent Gates part. It's really not clear though;
The website I've found it on, doesn't list the Gates hose as being for my car, it lists the previous Sasic hose. But it lists the Citroen part number agains the Sasic hose and then allows you to search by that.
Doing that turns up 3 possible equivalent OEM suppliers (one of which being the Sasic hose, another being the aforementioned Gates part). If you then click on the Gates part and then onto what cars that fits, it says it's not for my Xantia (for other Xantia models).
So next stop on the detective trail was the Gates on-line catalogue. That doesn't recognise the Gates part number that the site above gives me

Searching Gates online by my car, doesn't list any Gates hoses suitable for my car

(I'm thinking that's possible though, because it may have been the right part, but might not be a part they supply any more in which case it would have been removed from their current catalogue???)
Anyway, basically I'm going round in circles here...
The Gates part might fit, or it might not (looks right in the pics). Time to take a chance!
mister-auto catalogue says they stock the Gates part and it's only £15 more than the Sasic hose (but says it's not for my car of course

) and that it's in stock. Given previous experience, I messaged them first to ask if it actually is in stock - they've checked and computer says no! (TBH, I am actually very impressed with mister-auto customer service, far better than the wankers at GSF!!!)
So next stop, more searching on-line for the Gates part number... Which has turned up a supplier in Germany.
I've placed an order with them (at exoribitant cost and have then paid an extra exorbitant amount for express delivery) and we'll see what happens... a) if they actually have it and can supply, b) if it then turns up in good time, c) if it then does bloody well fit after all that!!!
What a drokking saga though!!!
If this fails, I think next stop will be to try and buy a top hose, cut the good quick connect end off it (same as the bottom hose) and then cobble that together joining it onto my original used bottom hose, just to get the drokking car working again!
