C2 means that their last years tax is in order.
is this not because their previous years tax were in order?
C2 registration... does not reference in anyway to any builders ability to do work or the standard of that work.
It is not related to your standard of work and for someone picking it out of a yellow pages having "C2 registered" as an advertising method does not guarantee anything
All you need to get a C2 is a letter of offer for a contract and a tax clearance certificate.