Hi Yorkshire-Rich,
I can see there is a problem here. Zurich, or any other insurer for that matter, should not be liable to pay for damages caused by faulty workmanship by the builder who failed to ensure the 'adequate support' which led to the crack widening. It is all to do with something called 'Proximate Cause' and poor workmanship is a policy exclusion and you should persue damages through a civil claims procedure if the party is still traceable. Zurich have re-instated cover on the basis that everything has now been sorted to a (presumably) satisfactory standard.
RE: Subsidence being on record. It does seem a little bit harsh that your property will now appear on the insurers database as suffered from subsidence. The actual definition of subsidence is "sinking of the Earth’s surface in response to geologic or man-induced causes; and where there is a sudden collapse of something into a hollow beneath it ". So, your perception of this may be different to others.
As you have contacted the insurer regarding this, and they have refused to remove this from the database, you will have to employ a RICS/FRICS approved surveyor to record his findings and submit a report to the insurer. The surveyor should also check for subsidence reports in the immediate area to assist them with your property. If the surveyor concludes that there is no evidence of subsidence then the insurer will have no other alternative but to remove this information unless they wish to challenge the surveyor's findings; which I doubt they would do. Note that the fees would have to be paid by yourself in a similar way that you would have to pay to employ a loss assessor to object to the insurer's loss adjuster. If the insurer wishes to employ their own independent, professionally qualified persons, they will have to pay the fees for that person.
If you do find that the property has suffered/is suffering from subsidence, you may also find it difficult with your mortgage company (assuming you have a mortgage); if not, then good-o, until it comes to selling/letting the property if this is your wish for the future.
In the meantime, you may want to try Adrian Flux initially or Towergate (if and after the property is underpinned).