Slipping cranks, drive chain slip?

Hehe, dingle speed bike!
Check for a cracked chainstay?

My guess would be rear axle! does the BB spin smooth without the chain and is the chain-ring spinning true?

Both suggestions are logical from yukirin and mule manning but cheapest is, check drive side wheel nut.

Thanks very much for these suggestions. I'll investigate and let you know how I progress. Appreciate it :)