new build. used tyvek breather membrane under tiles so we didnt have to vent loft space.. it is very wet on inside of membrane..anyone had similar problem.  Have not cross insulated ceiling within loft space  yet so maybe the loft is still 'warm'. Looks like i have paid for tyvek but am now gonna have to install vents to soffits also..

Is there a vapour barrier in/under the floor of the loft (example: plasterboard, VPC, ceiling joists, insulation, loft space)?

If you are still working up there a lot then it's possible water vapour is getting through the open loft hatch or holes for pipes? downlights?