If I happen to delete say group5 in the member groups list or cat id "alpha" in the categories list or board id "wr" in the boards list, I can no longer access these ids. Ideally shouldn't I be able to remake a board or cat with the same id? And shouldn't any new member groups I make take up the old numbers? Cos curently I have groups 1,2,3,4,5,8,9 - no 6 and 7!

Those boards/categories etc. are not completly deleted when you click the "delete" button. They're flagged as deleted and all access to them is removed. This is done so that if someone cracked your account and deleted everything then it could all be restored. After 14 days of being deleted, boards/categories can be purged (Admin -> Purge Forum Database) which makes their IDs avaliable again.

ok what about deleted member groups? Can I get those IDs back?

I believe that member groups are slightly different in that their IDs are all auto-increment ones. If you let me know the forum username I can reassign all the group IDs for you

I shall let you know by PM soon