I'm more of a beer man myself ...

M.

There is no single answer for this. Under IEC standards, for the TN-C system, the neutral is also an earthing conductor (PEN), earthed at multiple points, and should not be switched with a 4-pole CB. (however, under special conditions, the whole installation can be disconnected). Within the installation, the neutral and earth conductors become separate (TN-S-C), and then a 4-pole CB can be used. However, for TN-S systems and TT systems for example, there are conditions where the neutral can become live (especially in a large system), and an isolation is not complete without isolating the neutral. A 4-pole CB is the easiest method for this (or 2-pole for single phase). If a separate neutral link is used, it requires care to ensure that the neutral is never in a disconnected state when the phases are still connected. So basically, don't disconnect the earthing system via the neutral. 

I measured the voltages here at Kelly Towers I got 241 volts P to N 241 volts P to E and just 71millivolts N to E I know that I'm not on a PME system its an earth to the metal cable sheath the only N E links I know of is in a flat at the top of the road and at a new linkbox in the next street. I just thought ide post this because I'm guessing this is a common situation I don't think there's any danger from the neutral

Disconnecting the neutral may well be a practical consideration, over and above BS 7671, in installations incorporating RCDs ... quite simply for maintenance of certain circuits when other parts of the installation remain energized, whilst avoiding the "oops" moment when N and PE (or another part somehow connected to PE or Earth) make contact.

I'm not on a PME system its an earth to the metal cable sheath

The situation is typically a little more subtle than that - even though installations may be "presented" as TN-S, and may have originally been constructed as pure TN-S, any modifications or repairs in recent decades would almost certainly have linked N and PE out in the street somewhere. So what most people end up with is a PME supply but with your nearest N-PE link somewhat remote from your premises. If you ask the DNO they will likely say "PME conditions apply" and you should select bonding conductor sizes etc on that basis.

   - Andy.

You may have been there long enough to "know" the true situation. My N-PE link is under the side lawn, but the next owner of this property is most unlikely to know.

Perhaps I shall get my multimeter out tomorrow. There may well be a N-PE PD because there is usually some current in my earthing conductor.

Your right it does present as an earthed thru the cable sheath  there no N PE link  anywhere in my property. As I said there's a link in a flat at the top of my road  and in a linkbox  in the next street. So yes its almost PME but not in the full sense of the word. I posted my original comment just to point out what the situation was here as I'm sure it's the same in a lot of places especially with ancient streetmains