Heating system vent pipe question please help!

[Coughingchildren5]

Hoping pigletjohn is about or someone else with an understanding of how these things work!!
I have a system boiler, indirect vented hot water cylinder (with immersion heater), expansion vessel and cold water storage tank in the loft.
The hot water cylinder has a vent pipe which comes out the top and then winds its way up to the cold water tank in the loft. We are having work done and the plumber has said that this vent pipe was incorrectly installed and should be rising through the building rather its current route. So as the roof is also being repaired we are taking the opportunity to find a new route for the pipe to take so it can rise.
My question is, is this really necessary?? What is the point of the shiny red expansion vessel?
We don't have problems at the moment and it's been this way for years.
Anyone care to set me straight?!

The point of a new shiny red expansion vessel is that, your plumber can get more money.
There is an old saying. If it's not broke, don't fix it.
Just as your plumber exactly why he is recommending a new expansion vessel?
Yes, it is better taking a more direct route but, like I said. Not broke.

the vent pipe from the cylinder is not to do with the red expansion tank, which is for the sealed (pressurised) boiler.

It is fairly rare for a cylinder to vent, but this can happen if it overheats (maybe from an old and faulty immersion heater).

I don't know how bad the existing route of the vent pipe is. I suppose if it has horizontal sections it might be prone to airlocks or sediment blockage.

I'm glad you have not encountered any problems so far. The vent pipe only comes into operation if something goes wrong, it is a safety device. The number of people killed or scalded by these faults is very small (not zero), because the safety features are intended to protect against them.

Sorry if I wasn't clear, the expansion vessel is already there and fitted. The plumber wants to reroute the vent pipe.

Currently the pipe travels down under the hot water tank and along the bathroom floor into another room and then up to the loft.

My question is whether it is really necessary to go to the faff of rerouting this pipe so that it is always rising through the building.

Thanks Pigletjohn, so while we are making a mess it sounds like it is a good idea to reroute it. There are horizontal sections.