On failing her PCA and losing her IB my 45 year old client claimed IS subject to the usual reduction. Her 67 year old partner could have claimed PC instead, escaping the reduction, and on taking our advice did so with a request for backdating to the date my client had claimed IS.

PC has been awarded from the date of claim with no backdating, leaving them £11.50 per week worse off during the period that they were claiming IS instead of PC.

What (and where) is the rule about the exclusivity of PC and IS?

It's section 124 of the Contributions and Benefits Act, the primary provision for income support. This makes it clear that you can't get IS if you have reached the qualifying age for pension credit or if your partner is entitled to pension credit, which implies both being the right age and having claimed . There isn't a mirror image provision in the State Pension Credit Act as far as I can see.

The commentary in Mesher and Wood points out that a couple with one member each side of a 60th birthday can therefore choose which one to claim, since there is no entitlement to any benefit without a claim, and comments that pension credit will almost always be more favourable. I am trying to get my head round the idea of a retrospective claim for pension credit where income support has previously been in payment. Presumably the point is that once you have made a choice for a particular period that is irrevocable.

Brigid