Although the letter implies an intention on your part to jointly take on a lease with you ex partner, I don't think it amounts to a contract.
It is difficult to say without knowing exactly what the letter says (whether just you or both of you express an agreement to enter into the lease), but I imagine at best it would constitute an agreement to agree, which under contract law would be considered enforceable because it is too vague, ambiguous or incomplete to be classified as an enforceable contract.
The only thing I would be concerned about is if your ex partner relied upon your statement that you would enter into the lease and as a result incurred some major cost or expense. Then he may have a claim for in estoppel. But in all reality, this is an equitable issue that is unlikely to be taken to court for moderate sums of money.