Ok, so I know it's a joint family income of 40k+ a month, but what documents will I be expected to provide to prove it. Will a letter from my employers certifying the last three months salary be enough? Ditto for my wife?

The reason I ask is because my last month's salary wasn't banked but given to me in cash and so my bank book would likely show a missed month's salary if that was the proof needed.

i think you need an income certification letter from your embassy. so its up to what your embassy needs as proof of income..

So how does my Thai wife prove her income then?

So how does my Thai wife prove her income then?

Tax receipts

The last three months of tax receipts for any Thai income. For foreign income letter from your embassy and supporting documents.