Was questioning to lead the posts the correct way, I don't deal or have experience with HV so would be out of the conversation but if he's talking about running a 6mm 3core to his shed under his back garden then its all usually manual backfill and wouldn't warrant a sheath test as most domestic sparks don't have the appropriate tester to do so, no column on their test sheets for it either... wasn't questioning the testing of the sheath but the circumstances of the install if he's doing a sheath test as this tends to be past your domestic set ups thus curiosity got me, not often a test mentioned on here.
I bet you can drop this conversation into the forum and 90% of the guys who have buried cable on here will not have done a sheath test, I've not done it often myself but thats more to do with the fact I don't bury many cables. I questioned him because of his strange question at the beginning of using a blue pipe as a sheath - its one of them things that ring alarm bells, so had to clarify exactly what kind of install he's doing, he seems to do a test that most on this forum will never have even done yet asks a weird question to boot when standard ducting would be the way to go, even if he had a 10mm clearance he would not get that swa though that pipe as both would have natural bending in them from storage and the friction would be high over 15m length.
@OP - I wouldn't be going anything less than a 50mm duct to drag a 6mm swa through, it will also allow for future issues to be easily rectified if needed to change the cable.