Thanks. I found the quotes.
If fewer turbos are needed on a Sunday and they can use 4 units as opposed to 3 but there are enough turbos anyway, why can't 4 run during the week and make use of the sidings? They use to use the sidings in the past, when platform 7 didn't exist.
If it's due to the costs, what makes Sunday more economicly viable than Monday to Saturday?
I'm not sure how allocating an extra unit on weekdays will help with the current timetable - you would end up with each unit spending over an hour at Gatwick - is that really worth it?
Gatwick services have to fit in with the Thameslink timetable (and Southern trains to Reigate) so they have to run in the path
REI xx32 - RDH xx37-xx41 - GTW xx50-xx00 - RDH xx09-xx13 - REI xx17
and presumably the same on the other side of the hour - presumably using platform 0 at Redhill in the Gatwick direction and platform 1 in the Reading direction
REI xx02 - RDH xx07-xx11 - GTW xx20-xx30 - RDH xx39-xx43 - REI xx47
A later departure from Gatwick isn't possible because Thameslink run at xx03 and xx33. Reigate platform 2 is occupied xx23-xx30. The GWR service is also pathed in front of SWR stoppers between Guildford and Ash and between Wokingham and Reading. Given the position of everything else, the Reading to Gatwick service is probably in as good a path as it is going to get at the moment.
A 40-minute turn around at Gatwick doesn't work because you would need to find somewhere to park a unit at Redhill for 40 minutes as well to put the stopper on the other side of the hour and they don't have the long siding any more - so you go from 6 units to 8 units with no extra contingency at the Reading end. That just isn't how any kind of public transport service is designed to work.
If you look at RTT for this week, GWR lost six trains to Gatwick on Monday, two on Tuesday and one on Thursday. Only the ones on Tuesday were terminated at Redhill, the others either didn't run throughout or were terminated elsewhere. Timekeeping between Redhil and Gatwick otherwise looks quite good. It doesn't seem to me that the extra unit / contingency is needed.