I never quite understood why when wiring an out building it is not advised to use the main property earthing . In most cases the only option is an earth spike that is a dubious alternative?

I always find it amusing when the use of the concrete floor rebar as bonding/ earthing is mentioned. 
The chances of getting on site during the installation of the concrete floor in my experience is remote at best. Often the electricians won’t know that the job exists at this stage. Builders can’t even stop boarding over cables let alone involve us at the initial stages. 
Then there is the questions of the type of rebar used, welded or tied with bits of ‘bendy’ metal, what is the resistance of the rebar is it a reliable earthed structure capable of accurate results?
Using the structural steelwork is a better option, often with very low resistance and often exposed when the electricians arrive on site. 

This is something that is mentioned in EV charging, with the same questions, quality and availability of access to the rebar, I think may apply. 

It may ease your worry to realise that the rebar does not need to be welded to get a decently lo electrode effect unless you connect to a very short length. - the connections between the bars via the bulk of the concrete is enough, and the fairly scratchy connections where the bars cross, - so the twist and tie rebar is still worth connecting to.  It is  worth a play with a meter if you get the chance.

Note however that a concrete encased electrode is not the best idea for lightning protection - a really high current, far higher than a few hundred volts of mains could ever push through,  tends to crack the concrete.

It is a good idea,  and required in some countries regs, but not ours, to make two connections to the buried metal, so that  you can do a ring-round continuity test with an ohm meter between the two wires and have confidence that it has not rotted off or snapped below ground.

Mike

It may ease your worry to realise that the rebar does not need to be welded to get a decently lo electrode effect unless you connect to a very short length. - the connections between the bars via the bulk of the concrete is enough, and the fairly scratchy connections where the bars cross, - so the twist and tie rebar is still worth connecting to.  It is  worth a play with a meter if you get the chance.

Note however that a concrete encased electrode is not the best idea for lightning protection - a really high current, far higher than a few hundred volts of mains could ever push through,  tends to crack the concrete.

It is a good idea,  and required in some countries regs, but not ours, to make two connections to the buried metal, so that  you can do a ring-round continuity test with an ohm meter between the two wires and have confidence that it has not rotted off or snapped below ground.

Mike

Yes the idea of multiple connections would seem to be a valuable one. But as I said putting this into practise on any site other than a prestigious one, or perhaps one governed by an architect seems unlikely in my experience. 
builders tend to ‘crack on’ and let electricians worry about their job.