So the cws in the attic needs to be moved a few metres. Is it ok to run the vent pipe across from the existing pipe work with a slight rise to where the new position of the cws is?
Please tell me what the best practice is here?
Yes, it is fine to extend the cwt vent pipe. It has to be a minimum height above tank. There is a formula for this.
Just be sure to give the pipe a constant rise before dropping down into tank.
I would advise to use copper because plastic used as a vent pipe is very prone to being accidentally pulled out of tank and could cause a flood.
Clip and insulate pipe.