There is a good (and readable) paper on the SPSO website - http://www.valuingcomplaints.org.uk/sites/valuingcomplaints/files/resources/Querulous%20Complainants%20WEB.pdf
IMH experience the majority of querulous complainants have an underlying mental or personality disorder. Unfortunately this means that they will often be quite harassing to the people who are trying to help them.
They will often have a very definite idea of the answer they expect to get in response to their complaint and will reject any answer which does not support this already established expectation.
Their belief in conspiracies is often unshakable despite the relit y being that the public sector is just not competent or loyal enough to run a proper conspiracy.