'fraid I am not an complete agreance with Miss Zippy. Albeit she has gone through all the requirements for a legally binding contract, and I do agree that your only problem might be the 'intention to create legal relation', the courts do make allowances for this, and in practice, black letter law does not always come into play.
You can make a money claim on line - not sure what the amount you can go up to now, but it is more that the previously stated �5,000
In the first instance, why not send your ex a well worded letter from your solicitor as a pre-court procedure. But, you must be prepared to take it to court if this is not forthcoming - never make empty threats.
Money Claim Online - form the county court website guarantee to serve documents within 48 hours - timescales for hearings vary on your location - foir me in North Wales, it is currently about 5 to 6 weeks.
Good luck.