What tool do I need?

[DrPhill]

@all: thanks, this is good stuff - it is making me think carefully about each stage in order to get the best chance of success.
(1) Marking out - masking tape and careful line drawing

You may (or may not) find this tip helpful; to draw a line along a cylinder, you can use a small piece of angle iron. This will sit and "self register" on the cylinder, making the marking task easy. "Back in the day" rather precise rules, looking just like angle iron, but rather better made, were available.

BugBear

A good idea - I had not heard that idea before. Thanks

Yet another option which should work nicely on thin metal - a fine needle file - quite cheap and easy to buy..............

Another good idea - may be easier to source locally as well. Thanks