Replacing ceramic downpipe traps.

Frere Jacques

Did a lot of searching on this one but didn't find a whole pile on it.
We have two downpipe traps that aren't holding water and need to be replaced. I had a specialist look at them when we bought the house. He said structurally the ground around the drains are fine but he said it's only a matter of time before the ground is undercut and subsidence becomes an issue.
I don't have the cash to pay to get them done so I want to tackle it myself. I'm a toolmaker by trade so would be handy enough but I'm just a bit wary of digging so close to the house foundations. The one on the back is running to the sewer pipe and the one on the front corner of the house is running into a soakpit. I plan to dig out around the drains. Cut the old pipes with an angle grinder about 2 feet from the U bend and use a pvc coupler to join new pvc pipe onto the old lines maintaining the fall. I've read that some people back fill with sand/cement mix but other people recommend laying the pipe on a bed of gravel and backfilling with earth which is then compacted.
Can anyone give me any tips or what methods, materials should I use. The front corner trap is complicated by the fact it runs adjacent to the driveway which is an old tar drive which I don't want to undermine.
Any tips welcomed.
Jack.

rayjdav

Post up pic of the areas involved for more detail.