Thanks for all the replies. So it’s either the mech or the free hub, neither of which has been mentioned as a possible cause. If it is the free hub, then is there any reason why it only slips in the 1 gear?
Not looking forward to taking it back really as it’s been a struggle to get this far! Most of the time I’m being told either there is nothing wrong or what do you expect, it will slip if you ride it like that. All Trek dealers know about the issues with the free hub so surprised they haven’t thought of checking that.