I do mine in late october when it has dropped its leaves or late march at the latest. Once the leaves are off you can get a better view of the shape and the branches you need to remove.
I try to lay the newer stems as close to the horizontal as possible. this encourages the formation of flowers. hope this helps.

Official advice is that ramblers get done now - climbers, other than having excessively long growths topped or tied in place, are done end of February/early March.

is new dawn a rambler or a climber, I can never remember?

Thank you for the advice.

New Dawn is a rambler, Seaburngirl.