To think this client is being patronising....

[MeantToStopAtTwo]

...by saying, 'The money you earn working for me must be nice, Darling!'

(It might be if she ever actually got around to processing my invoices but that's another story...)

perhaps you should add here is your invoice please pay up "darling"?

"Oh yes, it will be when I get it!"

Accompanied by a sweet and sincere smile.

"Not as nice as the money you earn working for X, Darling"

Not sure if it's patronising (well, OK, a darling is always patronising) but it's definitely odd. No-one works for free, do they?

This client always, always calls me Darling at the end of every sentence. She is 13 years older than I am.

Start adding "dear" to the end of your sentences....

Trouble is Minty, irritating as she is, I really need to keep her as a client for the time being.

OK course if it were one of my clients what I'd really be saying is "prompt payment is always nice"

Sympathies OP

I was at the breaker's yard a few days ago getting a couple of new(ish) tyres fitted. I lost count of number of times I was called "darling" "love" and "sweetheart" but I had to grit my teeth as the tyres were such a good deal

Sorry OP but this thread bring this to mind:

Is she Zsa Zsa Gabor?

Is she really a client if she's not paying you though? Since she raised the issue of payment (albeit patronisingly) I'd seize the opportunity to discuss the invoices.