Hi Marmite,
Thanks for the data sheet. The best I have is an old Supply Authority Standard testing sheet which is very basic at 50 Meg Ohms. No Cable Length. I'm thinking if the three phase results are similar and over 5 Meg Ohms for cable length fifty metres for 5000V dc held for a minute then it will be OK to reconnect and retest in 12 months. If its around 1 meg Ohm for 50 metres replace as soon as possible. Under 0.5 Meg Ohms energise with eye closed.