There is engineering work at the Easter Weekend.  There will be no trains serving Charing Cross and Waterloo East stations on Good Friday 18 April, Saturday 19 April and Easter Sunday 20 April.  All trains will be diverted to Cannon Street.

The Southeastern website says:

Passengers for London Charing Cross and London Waterloo Ease can use London Underground services from London Bridge and London Cannon Street. Valid train tickets will be accepted for travel, in either direction and by any reasonable route, between London Bridge, Cannon Street, Southwark (for London Waterloo East), Waterloo, Embankment (for London Charing Cross) and Charing Cross underground stations.

However there is no confirmation that the Cannon Street Underground station will actually be open; it is usually closed on Sundays even when trains are diverted there.

On Friday there will be a Sunday-style timetable from Sevenoaks to Charing Cross Cannon Street.

It appears that trains are scheduled to be running to Waterloo East and Charing Cross on Monday.