If i did this then there is heating pipe work which is metal running from boiler houses that would be on a tncs earthing arrangement and bonded from that earth system running into another shed which would be tt earthing and bonded from that earting system.

That sounds to me that any PME potential would be transferred into the TT shed via bonding - completely defeating the purpose of TT'ing the sheds in the first place. You'd have to TT everything that shared extraneous-conductive-parts.

I presume there is a generator that will start and maintain the supply if the main RCD trips?

Wouldn't the generator normally be connected upstream of RCDs? Otherwise ADS is rather compromised if power is re-applied after a safety device has opened in order to disconnect.

   - Andy.

Yes generator is at origin of supply and covers whole site.  

And yes I was thinking there would be different potentials and perhaps leaving it worse than current state.  As it is an exsisting installation would maybe as in earlier comment best to leave all on tncs and make a note that is isn't to current best practice but it doesn't warrant a coding.  

If plumbing and other metallic services are common to the buildings you cannot really separate the earthing zones - as you say, they will end up some indeterminate voltage apart, and bonding removes that but it also restores the TNCs earthing,

Again, if it was being done from new you could specify plastic pipework, but then you could just as easily  specify  a welded length of stud-rod  to the re-bar grid in the concrete floor and keep the TNCs.

I've only ever done it on my own property, but its not hard to do any time before the cement mixer arrives. Actually so long as there is no DPC below it a large area of reinforced concrete in contact with earth is a surprisingly good electrode both for mains and RF (though the tendency to shatter the floor slab precludes the technique for use in lighting systems !)

Mike.

If plumbing and other metallic services are common to the buildings you cannot really separate the earthing zones - as you say, they will end up some indeterminate voltage apart, and bonding removes that but it also restores the TNCs earthing,

Again, if it was being done from new you could specify plastic pipework, but then you could just as easily  specify  a welded length of stud-rod  to the re-bar grid in the concrete floor and keep the TNCs.

I've only ever done it on my own property, but its not hard to do any time before the cement mixer arrives. Actually so long as there is no DPC below it a large area of reinforced concrete in contact with earth is a surprisingly good electrode both for mains and RF (though the tendency to shatter the floor slab precludes the technique for use in lighting systems !)

Mike.