For the benefit of others thinking of doing so, this is the kind of information you get:
So that's address, name, gender, date of birth, marital status and occupation. There's actually another column between name and gender that has the heading "O, V, S, P or L", but these are all blank on the pages I've bought and I've no idea what they mean. The obvious possibilities (disability, forces membership, nationality) don't seem to fit.
This doesn't amount to a lot, and for almost all my relatives it wouldn't tell me anything I didn't already know from other sources. However, in the particular case above, it does: it tells me when my dad's paternal grandmother, Laura Bartle, was born.
This is something of a breakthrough in my family tree, as Laura Bartle née Cooke has been holding out for years, refusing to divulge enough information for me to track down her parents. Her sisters, Rose and Emily (who was only ever known as Em) (or possibly M, like in James Bond) were complicit and also covered their tracks. Laura was 25 in 1896 on her Marriage Certificate, 31 in the 1901 Census, 38 in the 1911 Census (when her sisters were staying with her) and 78 in 1948 on her Death Certificate. I can't even find her sisters for sure on any Census before 1911 because I don't know when they were born.
Ha! Well I didn't, but I do now.
I knew from the Census information that the sisters originally came from Gloucester. One of my dad's cousins apparently knew why they left, but when I phoned him a few years ago to ask he was unavailable (it was the day of his funeral...). I did find a clutch of Cookes in the 1881 Gloucester Census who fitted the bill quite well: an Emily, Laura and Rose all living together at roughly the right time and of roughly the right ages. Unfortunately, the name of their father was Edward Cooke, a corn porter, and I knew from Laura's marriage certificate that her father was indeed a corn porter but his name was William, not Edward.
I nevertheless sent off for the birth certificates of the candidates to be Laura and her sisters, in case they gave the father's name as "Edward William Cooke" or something, but they didn't. However, that does mean that when I look at the 1939 Register and see that the dates of birth for Laura, Rose and (on another sheet) (for 56, Dragon Road — what a great address!) Emily correspond exactly with the ones on the birth certificates, I know they're right.
YOU LIAR, LAURA COOKE! YOU LIAR!
I started work on my family tree 15 years ago, and because you chose to give the wrong name for your father on your marriage certificate I've been stuck at you ever since. LIAR LIAR LIAR!
I also found out that my dad's recollection that Emily married someone with the surname Maude was correct. Whether or not it's also true that he was an American with whom Emily went across the US in a wagon train is not indicated in the 1939 Register.