I am running oil pipes under a gravel drive where people walk and cars
drive. I was going to put the copper pipe inside some spare MDPE mains
water pipe I've got so it should be well protected.

How shallow a trench could I get away with?

It's not (just) that I'm lazy, I'm also trying to avoid damaging some
propane lines which also run through the same ground. I will have to
cross them at some point. Hopefully they will be buried quite deep.

Cheers

Nick.