Time to get rid of those unsightly wires running down the wall. The existing cable is undamaged so would like to reuse but just wondering if it's advisable to run sheathed cable down pvc conduits (placed in channels cut a the wall) or should I be using unsheathed ? Will be 2.5 and 1.5 cable and 20mm conduit. Any maximum number of cables to run down a conduit (overheating risk ?). Thanks.

There is absolutely no problem with running the PVC insulated and sheathed cable in conduit, we do it all the time in the West.

Using singles may be slightly cheaper but the entire run will need to be enclosed.

Don't fill conduit more than 50%.

With the exception of your water heater and kitchen circuits which are often working near capacity I wouldn't worry about de-rating the cables as they will rarely get anywhere near capacity in a domestic environment.

One problem that I found is pulling sheathed cable through long conduit runs can be a real bitch. It will only be worse trying to pull old cable since the sheathing will have become more rigid and set and it will not be as flexable. I'ne done it but had to basically construct the conduit run and pull the cable piece by piece. I used external mounted wire runs with snap on covers when I added wiring to my house in BKK. looks better than bare wiring and not as much trouble as cutting channels and having to refloat the wall.

Just an idea if you like the aesthetic look of wood.

Rather than getting channels cut and refilling walls I covered external cables running down from the wood coved ceiling in wood, did the corner of the rooms too, matching the door frames and window frames, it makes it a feature of a room.