It shouldn't require anything, I have two Thorns with this style of BB and had a previous bike with something similar, they've never needed anything other than the bolts pinched tight. Not even super tight, Thorn's first version of this style of EBB had four bolts, I assume they dropped that because it wasn't necessary. I put a bit of copperslip on mine, to avoid galvanic corrosion, but it wouldn't stop it slipping, if anything it would increase the likelihood.
Sorry that's not very helpful. It's such a simple mechanical design, if the parts are within tolerance I can't think why it wouldn't hold. These are used on tandems, so it isn't likely you're just to powerful for it! Have you had the bike long? Has it always done this? Do the lugs on the split meet when tightened? I had that on a seatpost clamp, which is basically the same design. It's maybe not impossible that the insert is slightly undersized, if you have some accurate callipers, give it a measure. If you don't find a simple solution, maybe give Thorn a ring, it's possible they've come across it before.
Good luck.
