They can leak even with the right bolts, it's also really easy to miss the gasket hole and partially obscure the pipe...

I usually put a _very_ thin smear of RTV either side of the gasket, and thoroughly clean both mating faces before fitting these.

I was going to suggest the online version of microcat to find the bolt part numbers -> http://cat.lrparts.ru/ but for some reason it doesn't actually break them out - the image shows a nut!

I'll have a look at my home install this evening if I get a moment and nobody else chirps up.

Don't know the bolt size (and not about to take one off to find out!), are you quite sure they are non-original? because even the original ones are a PITA to fit.
When I swapped out my turbo I spent well over 2 hours trying to fit those bolts and had I not taken them off myself I might well have assumed they were the wrong size. The paper gasket that came with the kit was useless (even with RTV smeared on both sides) and it was a soggy mess after 20 minutes. Fortunately I was able to reuse the original gasket which was plastic. I mentioned the time it had taken me on this forum and it was suggested that I should have disconnected the pipe at the lower end first. This probably would have make attaching at the turbo end much easier and saved much swearing.

The 2 bolts that hold the wiring harness to the front of the cylinder head are the same size fwiw.

Bolts might not be why it's leaking anyway. Have you checked the flange on the drain pipe is still flat? They are known to warp, and will lead you a merry dance with multiple new gaskets and/or RTV attempts, and the like.

Old thread on the topic here: https://www.landyzone.co.uk/land-rover/turbo-oil-drain-pipe-leak.318732