I agree with Andy regarding the IR readings. What are the exact measurements of the insulation resistance reported on the EICR? Did the tests apply 500 V with all loads removed, equipment disconnected, and each circuit tested individually? You might need to carry out further investigation into each circuit individually. Depending on the extent and location of moisture ingress, it might be feasible to dry out the cable using a blow torch, replace only the damaged sections, or apply some corrective measures such as installing additional protection devices or earthing arrangements. However, these options might not be practical or cost-effective in every case, and recabling some or all of the circuits might still be required.

I find it hard to see the insulation being below 1M without causing problems somewhere. I suspect you may need a more experienced person to do the inspection.

I had exactly this fault with an industrial unit around 10 years ago, I was there to do the remedials. 'All lighting circuits low IR' was the wording. It was an easy fix, in fact no fix required, there were rows of fluorescent lighting down the unit, quite old, and all had a capacitor between L+N internally. The Inspector had clearly never encountered this before, had not disconnected the lighting before the IR test, and had concluded it was a wiring fault to cause the low IR readings.

Like some others who have made comments. I find the requirement for majority of the MICC cables to be replaced just doesnt add up. I have worked with MICC my whole career, and its pretty indestructable. Even when an open end is placed in a tank of water the ingress only goes so far, doesnt travel the full length. I suspect the knowledge and competency of the tester perhaps wasnt what it should have been. Im sure by the sounds of it there are hundreds of individual lengths of MICC  installed and doubt every single section had such a low reading. 

Where dampness has managed to get into a pot/seal, then drying out is possble and then reterminating.

GTB

A mapj1 says, it's a 25 year lease with a lease agreement to return the building at the end of the tenancy in a safe condition. I can't find any historic records to show what the readings were back in 1999 when the lease was signed. 

Thanks for your input everyone.

You've given me enough information to put together a case to prove the building is safe, as far as the MICC is concerned.

We've been doing our own Fixed Wire Testing for the past 20+ years and the old guys were content to sign-off the tasks every 3 or 5 years. It was the landlord who sent in their own EICR team (amongst others) to 'evaluate' the buildings condition, and it was them them who decided the place was just about to burst into flames and kill everyone within a 6 mile radius... Having said that, they also gave a C2 for 'no earth bonding and/or lightening protection' even though the entire building is riddled with copper straps, grounding points and a huge earth park...

just to put this out there I have installed micc cable in some very changing conditions including marinas, never had a cable take on water once potted off before its potted off yes it sucks in moisture like a teenager with a milk shake, as it was a lighting circuits I would imagine it was the line and neutral joined together. to earth test carried out and there could be leakage in light fittings.or connection boxes due to damp out side of the cable it self in my humble opinion if a cable is sucking water it would go to fault condition very quickly.