Hi Micky,
You already have your divorce so......all you really need to draw a line under the finances is to have a Consent Order drawn up.
Wikivorce in partnership with NFLP (a law firm) do them for £149.
See this link for more info
http://www.wikivorce.com/divorce/Services/Consent-Order.htmlOr you can get one done by a traditional solicitor - but they would probably charge a good deal more.
The main hurdle is that to get it for the above price you need your ex to be happy to stick to your original agreement and to be willing to have this drafted up into a consent order and ratified by a court.
If she (for whatever reason) decides she wants a new deal - the whole thing can become an expensive legal dispute.
So the real key is diplomacy and approaching your ex in the right way so that she is happy to have your previous 'informal' agreement made into a legally binding Consent Order.