I made up Shimano Zee lever and a Zee caliper (bargain) with some hoses from superstar . However, I went to put oil in the front tonight and made a real mess, there is oil leaking from the banjo and no lever resistance at all – the copper washer on the banjo doesn’t seem to seal it properly.

Ideas? Give up and call a professional…?

Is there just one copper washer? I’d expect one on each side of the banjo.

One on each side

cross threaded?

rock based install procedure?

Aye, usually 2 copper washers per banjo

2 copper washers, torqued to spec, no rocks, don’t think cross threaded

Suspect solution is to buy shimano hose

Possibly. Shimano use rubber O-rings to seal, probably more tolerant of surface imperfections, but make sure they’re properly located in their grooves when you’re tightening the bolt as they’re very easy to pinch and damage.
Edit- being an aftermarket kit you’re using, is the banjo bolt the correct length? Too long and it may bottom out before it compresses the washers.

Take a flame to the washers, heat till cherry red and immediately quench in cold water

That will anneal them and give you a good staring point

Banjo bolt length is OK. Just seems in this case the crush washer isn’t the right solution. Based on a bit of searching, not unknown. Looks like shimano kits it is.