Daniel,
I have suffered leaks even though I have tightened up the banjo bolts to within an inch of their life. On an XK150S there are 4 banjos that have magically to all be aligned. Here is my secret ritual …
First, make sure you are using new fibre washers. If not, then reface old ones by rubbing them on fine sandpaper that is laying on a flat surface.
Check that the face of the lids that the banjos bolts screw into are flat and free of nicks, etc. Dress with a wide fine file if needed, or just to be sure. Naturally, use file only once lid is removed, so you can wash away all fillings.
Check that the mating faces of the fuel pipe banjos are also flat - they may be dished because of over tightning, or be nicked. Dress with wide fine file. This filed the nickel plating off my banjos, but it was needed to get them flat.
Check the outer sides of the banjos. I have found old white fibre washers hiding in the recesses, almost invisible, that needed picking out. Check the recesses look to be free of nicks, but not a lot you can do here.
Check fit of new/refinished fibre washers in the recesses. It is possible the outside diameter is too large, in which case sand/file down to fit snugly. Remove washers for now.
Next fit lids, but tighten their bolts so lids can rotate. Grease threads of banjo bolts - this makes fitting so much easier! Trial fit fuel pipe without washers. Screw in banjo bolts barely finger tight. Lids will rotate to optimum position.
Now check that the mating faces between the lids and banjos have no gaps. Gaps happens if relative to each over, faces are not vertically alligned and or spaced correctly. Likelihood is there are gaps and this is cause of leak. The remedy is to bend the fuel pipe untill all surfaces mate perfectly. To bend you have to remove pipe and hold, say, one banjo in vice (with aluminium keepers, of course) allowing you to use hand strenght to make an adjustment.
The key point is that the banjos should accurately line-up without relying on the banjo bolts forcing them into alignment.
On the XK150S, this is a long and tricky process, and takes patience - trial fitting, removing, adjusting, refitting, ad nauseam. Freeing the pipe from the carbs is like untangling those metal puzzles.
When you are satisfied that you have all banjos mating near perfectly with the lids, then you can do the final fitting with the washers. Again, greasing threads makes tightening smoother- and less torque will be needed. Tighten banjo bolts with your Whitworth spanner, but only just nipping them up. Now you can tighten lids up, and then back to the banjo bolts for final tightening.
Follow others guidance for how much to tighten the banjo bolts. Once you run up the pressure with the fuel pump you may have to tighten them a bit more if they weep. If you think you are using too much force, then go back to the trial fitting stage with no washers.
I have also sprung leak in a joint between banjo fitting and the fuel pipe, so look out for that being source of leak. This requires joint to be resoldered, then back to trial fitting stage.
After much patience and divinity, my system is fuel tight, and I didn’t have to apply too much torque, but probably more than others recommend though.
Regards and good luck,
Clive.