Does anyone know if there's a way to re-instate a forum topic which has been deleted? so as to re-use the exact original  URL?

If I re-set up a new topic with the same title, the forum number will be different,  i.e original url was like;
.com/index.php?/forum/5
whereas if I re-do it, we'd now get url like .com/index.php?/forum/10

thanks in advance - for any info if / how to do this

There is no built in way to do that, no. It could be done in theory modifying the database directly, which we would recommend against.

Do a 301 redirect with a regex match to the new forum number.  Would that work for you?