Dave,
What you say makes complete sense, but I think for a simple account like window cleaning it would be a bit of an overkill and we can always plead ignorance, since what you're basically saying is that we should be doing 'double-entry book keeping' (one side for cash, the other for bank; anotating running costs in red; etc).
The way I do my books is that I account for what I've cleaned; not what I've received in cash.
Say for example, I earn 100 quid, but only receive 30 quid, I put in my books that I've taken 100 pounds (regardless of it being cash, cheque or not yet received).
The money always comes in, apart from an odd exception, and for that I write a short note explaining the 'bad debt' and use that as a tax deductable expense.
This may not be the correct way, but I wouldn't be too worried if a tax inspector looked at my books.
I would be worried if he looked under my matteress though!
