Cupboard under the sink is very smelly, same smell briefly comes out of the sink after it’s been drained. Thought it must be the dishwasher pipe as it’s in before the trap but it doesn’t smell. Took all pipework of an cleaned (wasn’t dirty tho) an smell still present, u bend was full of water so working. Took kickplate of an no smell  under units.

pipe goes into a 110 pipe in the floor then out of building, only thing in that leg of sewer is kitchen sink an dishwasher. 
As it’s smelling In the cupboard all the time does it have to be the pipe before the u bend causing it? Crack or bad seal?

Yes must be connection between 110 and 40mm pipe

That smell is in my brain while I read your comments.   I had this at a wc connector. It was a sealed joint and extra mastic so I didn't suspect it for ages, but eventually noticed a tiny gap almost out of sight. New plumbers mait all round and thoroughly packed: sorted.