Yes, I have all that info, thank you!
His birth certificate says he was born on 13 January 1901, which was a Sunday. His mother registered the birth on 23 February, a Saturday, which is 41 days after the DoB that she stated, and presumably the last working day possible for registration, the next day being a Sunday, and if the maximum time allowed was the same then as it is now.
All documents I've found for him so far that have a recorded DoB say 13 January, except for one. His school admission record from 1911 says 10 January.
So it's just made me wonder whether she might have actually given birth on 10 January, but told the registrar the 13th, because otherwise she might've been fined for late registration, albeit only a few days late.
I believe he celebrated his birthday on the 13th, so it is probably correct, but it's just interesting to ponder!