Brake or fuel?
 

I don't know what this is but it looks pretty crusty. It sits near the fuel tank on the drivers side.
Any advice would be much appreciated.

Dosing pump for the FBH. Does look crusty, but it may still work :-)

Hi Simone.
Unplug it and disconnect the fuel line, then remove from the car so you can clean it up, once cleaned up give it a coat of black paint & re-fit.

https://i.imgur.com/9WuzdFGl.jpg1

https://i.imgur.com/FAlEEBfl.jpg2

Scrap off all the blown rust.
https://i.imgur.com/ClNIANbl.jpg3

https://i.imgur.com/e9GOeNDl.jpg4

Use wire wheel to clean the rest.
https://i.imgur.com/oVJcWXil.jpg5

Do the same with the pump being careful round the in-let & outlet tube ends these should not be disturbed.
https://i.imgur.com/sMObYFul.jpg6

https://i.imgur.com/mj3ZXYel.jpg7

https://i.imgur.com/xnhedbTl.jpg8

Spray it or paint with Hamerite and fit it back to the car.

https://i.imgur.com/RUsbqcol.jpg9

https://i.imgur.com/bixCRD0l.jpg10

The above should give to a rough idea

never had much success with a wire wheel, shot blast it, much easier.

por15 paint does a good job protecting from rust.

All diesel owners with a fuel burning heater should check the dozing pump and bracket for rust as they are very very prone to rust.

macafee2

I would be more concerned about the inner sill by the look of things!