A couple of things I've noticed:
1) Not all of the pieces/items are uploaded. Searching for my great-grandparents, I had the luxury of knowing exactly where they lived, thanks to the 1939 electoral roll being on Ancestry. I found they did not come up using names (in various full and vague forms). So I searched via address, and whilst the door number shows on the list, it errors every time I try to click through. So, I tried searching for neighbours and every single one of them does not appear, until further up the road. The road itself goes from 1-23 (odds) and 2-42 (evens). The next piece shows 30-42 even then 23-5 odds. My rellies were at no.2 !!! So using Dawn's logic, I found that the third section of the reference (in red RG101/1234Z/
123/01) is the item and by adjusting it by one you can see the previous or following item along the walk.......except the one I need errors

Which brings me on to my next thing...
2) A household doesn't always fit nicely on to one document. In doing the above, I found husband and wife were on different pieces. If you're tracing them (or similar) that counts as two households to unlock

3) On the subject of missing pieces, my mum and her family are "hiding" (hence why I was searching Point 1 above, just in case). There are four of them in the household and all are deceased (although my Mum is too recent to show). Despite searching using all combinations of firstname surname, day month and year of birth, NONE of them show. Trouble is, despite knowing where they were, I can't search by address, as they were living in flats and the road doesn't even show in the index!! Arrgghhh!!!