What is causing short trains between Salisbury and Waterloo and when will it end?

Every day this month the 7.19 from Overton has been formed of 8 carriages instead of 10. SWR suffers no consequences for this 20% reduction in service because they pay no compensation. It makes 3 previously commutable stations un-commutable.

Why is this happening? We have had no warning or explanation (like that we have enjoyed for engineering work by Network Rail). We have no indication whether this is a permenant change or when it might end.