" have been told to lay board down on top of the floor boards in the bathroom. few qs 1 does the board have to be in strips?"
No it doesn't, the board is there to give you a level surface to lay the lino onto. Without the boarding the gaps between the floorboards would soon be visible as depressions on the newly laid lino.
" 2 does it have to go under the skirting board?"
If you are only laying lino up to the skirting board then the boarding only needs to go up to the skirting board, don't make the job harder if you don't have to.
" 3 previously I had carpet in the bathroom with a join (metal strip) joining it to carpet on the landing. what happens here? do I place board right up to the join / carpet?"
Personally I would lay board up to the carpet join, that way the strip you have will still be usable on this new join.
Mark