chunky munky wrote:

The Current Off Peak Piccadilly line service is
every 20 mins Cockfosters - Uxbridge
every 20 mins Cockfosters - Rayners Lane
every 5 mins Cockfosters - Heathrow
every 20 mins Arnos Grove - Northfields

In the peak the Northfields trains, more or less run to Ruislip and
there is a 5 minute service, both to Heathrow and Rayner's Lane.

The new timetable starting in January ("Shadow Running" when T5
trains start running, empty from/to T123) I understand will be every
10 minutes to T4 and every 10 minutes to T5, via T123. I think that
the T4 trains will then reverse at Arnos Grove, with no more
Northfield "locals". There shouldnt be a change to the Rayners
Branch.

That's a drop from 21tph to 18tph through the pipe, and from 18tph to 12tph
at Cockfosters. I suspect you've got something wrong there.