............But the relative got help, from the council I think, to sort out their benefits, pensions etc when the spouse died.
There is case law around this.... I am not in my office so can't point you in its direction, but I would suggest that: if you really like a fight and were not bothered about being palmed off by the council who will tell you that they can't back date a claim and that arrears aren't possible (without knowing what either of these terms actually mean), then I would get some advice (if you can find an adviser who knows what these terms actually mean! If you can get hold of a copy, the CPAG Welfare Rights handbook is an excellence source of information. Your local library- remember them, should have a copy) To be honest, it would be a long fight especially if this was some time ago and your chances of getting anywhere are limited.
Also, you need to remember that Council Tax Benefit was replaced with CTR in 2013. This should leave a pensioner no worse off than before (CTR is a joke overall: its not a national scheme so local councils can basically do what they want and from my experience, some of them treat the scheme as another way of hurting the poor). However, a new piece of caselaw I saw yesterday DG v Liverpool City Council [VTE, 4310M140277/CTR] may also cause you issues. While its not the same as above, there are some things from this case which a good council worker would use, namely:
" ‘Official error’ was relevant to council tax benefit, under the regulations. Council tax benefit is no longer available, having been replaced by council tax reduction in 2013. Therefore official error does not apply to council tax reduction." I can see this being challenged but that's how it stands currently.
Frankly, unless you have a load of paperwork and proof of what you did, when you did it and to whom, you may be on a fight to nothing.
Edit to add....if you like a bit of light reading with your morning toast, try this....
https://www.valuationtribunal.gov.uk/wp-content/uploads/2016/01/council-tax-guidance-manual.pdf?mc_cid=94058a397b&mc_eid=d46fc48357Further edit to add and now at my desk.... see Part 2 of Chapter 54 of the CPAG WR handbook, especially pages 1280 to 1287 from this years book. However, as I said above, CTB no longer exists, so you might have to look up the regs as they were at the time. Your local library or local Citizens Advice should have them. I doubt they have changed much, but this section doesn't mention CTB/CTR for the reasons given above.