I think I have read this in recent posts and that not only is it possible but, arguably desirable, being aleady in the right place to do it (ie the Immigration office).

When I look at the TM 8 however there is that section about providing details of the planned departure which for myself is not yet confirmed. Does this therefore preclude getting the re-entry permit or can those fields be left blank to be completed later?

Thanks in advance.

(I know there is also an option to get a re-entry permit at an airport.)

No, there's no need to fill that field, always wise to get at least a single just in case urgent departure is needed.

Some offices will allow you to leave the dates blank. If not you can put any date you want because it does not appear on the re-entry permit.

Just got a multiple retry permit from CM immigration last week leaving those fields on the form TM 8 blank, easy to do took about 1hour.