I have read conflicting reports that one needs to show 20k in cash on entering thailand from savannakhet on a non immigrant o based on marriage visa. I understand this may be required for a tourist visa but am unsure about a non immigrant o. If so, is a thai bank book with the last entry 15th June (if entering Thailand on 30th June) adequate or is it not necessary to show?

The 20K requirement is for any type of visa; but it’s highly unlikely a non-immigrant visa holder would be asked. 

 

A bank book is no good. It needs to be cash (doesn’t have to be baht) or travellers cheques.

Not sure where your heard about the crossing at Mukdahan/Savannakhet asking for the 20k baht. 

I have seen no reports of it happening when entering with a non-o visa or for that matter even a tourist visa.

I was in Savannakhet week before last - no such requirement. I have never been asked for the 20k at Nong Khai nor at Savarnabhumi.

I've re-entered Thailand from Savannakhet 3 times - in a roughly 3 year period - on an SETV from the Thai consulate there - showed UK bank statements on application, never asked for any cash.