Dave Ward, Route managing director South East route wrote:
In the last week of August we made some significant changes, involving the replacement or amendment of thousands of cables and hundreds of electrical switches. Every change was tested thoroughly to confirm the safety and reliability of the system. However, since then there have been problems with the existing, older systems and some isolated incidents of components failing.
So they put in new parts, tested that they all worked, but didn't make sure the changes were fully compatible with the existing systems.
There is no doubt that the reliability of trains and signals in and out of London Bridge lower station has seen significant degradation since the works in August.