Just had a logical thought, Space is already a premium at Hornsey so regardless of a potential 717 failure, one unit could be replaced with one another thus some 313's might just be moved to sidings on the route - only logical location would be Hertford North. - any bettings on how quickly those units will be vandalized if outstabled there.
No need to bet, it's a sure thing that any unit's left up there will be vandalised as the security isn't the best in the world and I'm referring to the fact that the fencing and lack of proper security lighting makes it a easy target.
Indeed a few years ago, a unit was stabled in the down siding one night for around half a hour before the next service terminated to couple to it and it had been vandalised so doesn't take long for the undesirables to target them.
I know one of the sidings in the yard there has been out of use for a long time now, no idea why so maybe you might know why?
Looking at Open Train Times and Traksy, it appears that Sidings 3 and 4 are both out of use today so that further impacts on stabling space there, anyway if memory serves me right if Hertford doesn't have any issues stabling rolling stock it should be able to cater for 8 Class 313s in the Up Yard and 2 Class 313s in the Down Siding so 10 units altogether but that leaves no room for any shunts or stabling during the day.
What about Hitchin? I know you've got the Up Yard there which has been used in the past to stable rolling stock, not too sure on which sidings there are electrified though or how many units it can take.
It's also a pity that the Up Yard at Welwyn Garden City wasn't upgraded to provide stabling as that could have meant expanding the facilities there with 12 car sidings on the Up Side which would have been helpful as I'm not sure if the Down Yard can take 12 cars or not?