You need to be prepared to submit your Witness Statement by 12 June, but so does the claimant, and ideally you should wait until you receive their WS so that you can respond to it.
On 19 June you should ensure that the court fee has been paid, in the absence of any notice of discontinuation.
Your Witness Statement is your story, but note the requirements as stated in the letter you have posted. Facts, paragraphs, clarity.
DCB Legal usually discontinue, and in this case they may do so by 12 June so as to avoid the need to submit their WS which will cost them money, in which case you will not need to submit your own WS. But it’s worthwhile your thinking about the structure of your WS in case they don’t discontinue, and you shouldn’t miss the deadline.