if the engineering work originally planned for last weekend was the same as this weekend, the line was closed Salisbury to Gillingham (and Warminster), with an isolated SWR service running between Gillingham and Exeter. So the implication is the allocated units were stuck at that end of the route for all day Sat and Sun, ie it’s nothing like the normal weekday situation...
Isn’t normal practice when a line closure blocks access to fuel, that extra fully fuelled units are sent, even if they are attached to last service train (as longer train) to avoid special moves.
If there were other units with fuel sitting in sidings all weekend, sounds like incompetence especially as it is not isolated line, but they could have got someone to take some units (even one train of 3 units) via Basingstoke, Newbury and Yeovil (or via Weymouth) to access from other end.
I don’t think the excuse of couldn’t get any units with fuel to Gillingham-Exeter section is watertight.