fr0d0
It's pretty important to remember that the only person whose assets are directly involved here is the patient/relative. It is important also that if they lack capacity, there is, or should be, a deputy/PoA holder, which I don;t think you state here.
If the individual lacks capacity to manage their own affairs, and a deputy or PoA holder has been appointed, then the latter is obliged to either complete the assessment or to decline (formally) to do so stating why. If the issue is that the needs have not (yet) been determiend then the LA has (at that stage) no legal entitlement to the data. Once the needs have been determined as falling within the scope of the National Assistance Act, the LA is entitled to charge and the necessary regulations create the relevant statutory gateway required by the Data Protection Act for the relevant disclosures to be made.
If the issue is something else, then the LA can do several things. The two most likely are first that if there is real property (i.e. land or buildings) owned by the person concerned then it can register a c(iii) Land Charge under the Land Charges Act, and the debt is colelcted when the property is sold or transferred. It could also seek a court order over non-property assets. AS Nitram states, the LA can seek a Court order for discovery of assets: give the existence of the powers in the National Assistance Act, they would almost certainly get this.
Note that the deputy or PoA holder could find themselves personally liable for any costs/losses incurred by the resident/patient as a result of their (in)action, but this is an area for specialist advice. Or the LA could apply to the Court of Protection for a new Deputy/PoA holder to be appointed. The latter is (in may lay view) very unlikely. The Deputy POA holder can always ask the LA for alternative arrangements, but the LA is not under any obligation to comply.
If the person lacks capacity, and no Deputy has been appointed (and by definition there is no PoA), then there is an issue of how the person's assets are being managed. If one isn't appointed, the LA can (and I have posted elsewhere on TP in a case where this seemed to be happining), but it will normally encourage relatives or others to get themselves appointed as deputies or appoint one.
Ultimately it is simple: the deputy/PoA holder is under an obligation to complete the assesment when requested and the LA has the right to request the information in accordance with the National Assistance Act and the relevant regulations. If they don't then they could become personall liable for the consequences. In either case, specialist legal advice is necessary - vital - if you want to contest this.
W