Well firstly you need to sort out the earth rod. If the rod itself is corroded significantly (presumably a copper-clad steel one that has lost its plating?) then it should be replaced. If the soil is corrosive (e.g. quite acidic) you might need to look at a solid copper rod (more expensive) or a stainless one for acceptably long life. For good all-year behaviour you want it deep, so if you can (and have checked there is nothing at risk) then driving in a 2.4m or so rod is better than two 1.2m ones in parallel, etc.
As a TT system it must have an RCD
first, before any thing like a fused-switch, SWA cable, etc, that could possibly result in a fault to earth. That could be in the main CU is you have taken sufficient care that the tails are protected against damage against the metal box or external forces that could move them inside. Which you should anyway!
What is in the exiting CU?
Depending on the outbuilding needs, it might be that taking it from a MCB in the main CU is OK. True you won't get as great an independence of fault handling, and it is hard to get good selectivity with two MCB in series, but it may be acceptable. If going down that route see if you can get away with a 40A D-curve MCB feeding the SWA so the instantaneous magnetic trip at the source has as high a fault current as practical.
If you do want independent operation of the two CU then as you suggest you could split the tails and put in a RCD then fused-switch for the outbuilding SWA feed. The main CU needs an up-front RCD as well, but that could be a replacement for its main switch if available.
Any pictures for us to pontificate over?
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I had it in mind , based on past experiences and the BBB, that the rcd had to be first, then split.
The RCD must be before anything that could reasonably have a fault mode to earth.
If you have the tails split with a pair of Henley blocks on a non-conductive panel such as plywood, etc, with the various cables pinned on it for mechanical support that is not likely. Then you can have your two 100mA S-type RCD if you want independence of operation.
However if you are looking at both down-stream CU with all-RCD protection based on instant 30mA RCD/RCBO then you should not see the main RCD trip outside of a major fault (SWA cable damage, downstream RCD failed to operate on significant fault, etc) or a lot of leaky loads over several circuts adding up to more then 50mA or so. In that case you could have a single main RCD and split after it for the two CU.
Other folk on this forum have also pointed out that the old plastic CU were better for TT installation in the sense of less risk of an earth fault! But obviously no longer acceptable due to the lack of fire containment.
On the positive side, the new metal CU and fire aspect has brought in the three-hole cable glands as a seal for the tails entering, and they are much better generally then a grommet as they also provide come cable movement control. I'm also guessing the more flexible 19-strand tails would be better as well as less likely to apply forces to the inside of the CU, and obviously good practice to clip the tails in place outside of the CU unless it is a very short run to the meter.
