As I have only ridden through Reading on a through train, I can only give the answer as Internet allows me.
But from the lack of information I would say that the parking is not safe enough to leave an expensive bike for 3 days.
The railway station information page gives 'stands' as main option for parking the bikes, the car parks are given as extra without any information on bikes, as you have found for yourself.
No bike blog about safe parking but there is a photo map where they show a missing (stolen) bike and mention how usual stolen bikes are in the location.
An English blogger who lives in the Netherlands and is very much aware of cycling and cycle safety mentions in one of his blog entries:
The new parking takes the total number of bicycle spaces at Reading to 160
which is 160 spaces and no mention at all about safe, under cover or secured places.
I take this as negative proof, no mention about secure bicycle parking, not even on those sites that would mention it when it would be there. Yes, you can park your bike in a car park, but knowing security in car parks, I would not rely on that, anything smaller than a car can be taken out without ringing bells.