Between 29 to 32 lb-ft...(39 to 45Nm)
But it's more important to pay attention to the new bolt specs than to just presume...Plated Brass or Steel?
IF using the old bolts, then new copper washers are needed - no matter what.
You're from UK, so the Newton/Meter measurements are used, while in the US - about 30lb/ft for the Backyard mechanic.
Due to the castings of the caliper body, it's easy to strip out the fitting using too much torque.
Clean the flange areas of the fitting, Bolt flanges and the lines' surface too - even body hairs and pollen dust can make your day. IF you can, use a simple caliper measure tool and see if the surfaces are plumb - if not throw them out and use new. The Caliper tool can be finger pressured - it's scraped (dragged) across the surfaces to see if light shines thru any of the surface or a burnished effect is evident to even to show the dimple, if so, the seal might not take to this surface - you should throw it out and use new so the seating dimple on the line of the bolt - so it is not suspect as to causing the leak.
So to me, to use "enough to keep it there" plus an 1/8th turn is enough for me.