I've just been googling around for a length of 12mm hydraulic hose and it all seems to be sold in imperial sizes, is that the case? Closest seems to be 1/2 inch, or am I missing something?
safest bet is to take one of the fittings out, bit of hose and take it to a hydraulics place, and the lengths / fittings ends you need. They can make up hoses to suit... some like Pirtek, that way you know what you're getting. Not worth messing about they're not that dear, especially if you have the swaged end fittings.
12mm ID will be 1/2 hose, it’s just a matter of what fitting you’ll need swaging on! That’s when the trouble starts! Any idea of the fittings or a pic?
Hydraulic hose sometimes has DN10 or DN12 on it which is the metric bore size in mm but the same hose can also have 3/8" or 1/2" on it as well! The actual size is usually in inches as well as the fittings. But the outer diameter can vary between hose manufacturers so ideally for a gates hose you should use the corresponding gates crimp on fittings.