I've done several bank recs on my accounts and while the payments are exactly right in terms of bank transactions I want to delete a some payments so I can allocate them to different invoices.
Is it possible to undo bank recs so I can do them again once I've sorted everything out?
Depends on the software you are using - but yes you should be able to either undo a bank rec, or with some software you can unallocate them and then reallocated them.
I would suggest you try different ways of doing it before you delete the payments.
Which software are you using? With Quickbooks you can undo the last reconciliation you did, but not further back. If it's Sage you can go into File - Maintenance - Corrections and open the entry you want to un-reconcile and then un-tick the box that is marked Bank rec on and it will be un-reconciled and you can then delete it and re-enter it against the correct invoice. You will then have to go back into the bank reconcile window and re-reconcile the transactions.
I would suggest taking a backup before doing these corrections so that if it goes pear-shaped you can restore the backup and start again.