Better than you'd expect for an established fleet and a tall order for a new fleet to achieve!
In the latest Modern Railways reliability survey, after a few periods of stagnating at around 3.5-4k miles per failure (known as 701D), reliability improved to over 9k miles per failure in Period 4 (four weeks starting late June).
The issue seems to be the very slow progress in clearing units for service.
Very true - its is a good reliability figure even for a bedded in fleet.
I will call that very thorough acceptance testing, which is a respectable way to do things

.
(my bold).
Just to keep the testers busy. Next delivery to Wimbledon from Long Marston is in the system for tomorrow evening - Wednesday 29/10/2025.
Real-time train running information for 5Q70 1435 departure from Long Marston Gbrf to Wimbledon Park Depot Sdgs on 29/10/2025. From Realtime Trains, an independent source of train running info for Great Britain.
www.realtimetrains.co.uk
5Q70
14:35 Long Marston departure
18:52 Acton Mainline
18:58 Willesden (low level)
19:17 - 19:22 Clapham Junction Pl 6
........ (round the Kingston loop)
20:24 Wimbledon Stn
20:31 Wimbledon Park Depot arrival
But the deliveries
usually run early. Allow an hour so I usualy aim for Wimbledon knowing I can catch it an hour early at Clapham Junction !
For those wanting advance notice the light loco from Leicester is
0V70
08:51 Leicester departure
12:31 Long Marston arrival
Real-time train running information for 0V70 0851 departure from Leicester L.I.P. to Long Marston Gbrf on 29/10/2025. From Realtime Trains, an independent source of train running info for Great Britain.
www.realtimetrains.co.uk