I'm afraid the days of spare duties for train crew have long since gone. (Back in the days of British Rail.) SWR and all other train operating companies are here to make a profit for shareholders and pay the DfT. Providing a train service is a means to an end.
Many years ago I did clerical work assisting traincrew supervisors and from memory levels of sickness were no higher during school holidays.
Hi Owen, we try to cover all absences where possible. In this case, it was not possible for us to locate a contingency member of staff to cover this service. If this caused you a delay of 15 minutes or more, you may be eligible for delay repay compensation. Please visit our website for more information and to apply http://bit.ly/2iVGozW