The OP wants to break of the return leg of the journey.
Break of journey is allowed on either or both legs of a Crewe to Inverness journey using an Off-Peak Return ticket, but the rules are different for each leg.
The outward portion of this Off-Peak Return ticket is valid for travel on the date shown on the ticket and an unlimited number of breaks of this leg of the journey are allowed on that day. Where a passenger wishes to stop overnight or the journey cannot be completed within one day, break of journey for an overnight stay is allowed. Where a journey does continue into the next day, travel must resume before 1200 and the time restriction applies: "by any train except those timed to depart Mondays-Fridays before 0415". No further break of this leg of the journey is then allowed except for the purpose of changing trains.
The return portion of this Off-Peak Return ticket is valid for travel until 0230 on the day following the last day of validity (i.e. one calendar month from outward journey). During this period, an unlimited number of breaks of this leg of the journey are allowed. If a passenger is still travelling at 0230 on the day following the last day of validity, they may stay on the train to complete that part of the journey but no further break of journey is allowed and all travel must be completed by 1200. On resuming the journey after each break, the time restriction applies: "by any train except those timed to depart Mondays-Fridays before 0415".