It's time to fix our wobbly fencing. I could just replace the panels
(after repairing one of the rotted posts with one of those metpost
repair spurs), but we'd like a gate in the fencing. Of course if I do
that I'll need to take out the old posts and put new in in different
locations.

The problem is that there are two sewers running under the fence. I
have the plan that came with the title deeds showing Foul sewer
(adopted) and Surface water sewer (adopted), but that's just accurate
enough to tell me that they are in the area.

We were planning to dig down 750mm (exposed to strong winds) and
concrete wooden posts in. What's the best way of digging the new post
holes and not damaging the sewer pipes?

Should I use those concrete-in metposts, as they only go down 450mm?
I'm worried that they won't be stable enough with a 5.5' feather edge
fence on top because the wind hits it square on.