Doh! Good spotting, thanks for pointing that out. I missed it when I first played around with this many months ago, and didn't look closely enough when I came back to it recently, trusting past me too much.
Holyoake became Prime Minister for 2-3 months before the 1957 election, which Nash one. Then Holyoake won the 1960 election (and several more). I didn't realise we'd had a repeat Prime Minister post-war, so nice to learn something new.