I would use the approach Nick sets out from the caselaw (apart from 12 months becoming 3, the only way in which the regs have changed is to remove the double bubble effect of the original version – that could take the date of claim back 12 mpnths to GC claim date then a further 12 months from that!)
The whole point of the SPC date of claim rules is to give pensioners up to 3 months to make a claim. In this case, as he has stated a date within that 3 month period, he has just not used all of that 3 month provision!
Pay as claimed/requested but do try to ensure he has put the correct date and/or that he knows you can pay from an earlier date if he corrects it.
