Is next month your first appointment or FDR? A judge may be able to sign off an order you've agreed at this hearing but they won't be making financial orders without hearing evidence at a final hearing, which is a very lengthy and costly process. It would be wise to get advice from more than one solicitor, going armed with a page of the facts of your case and a list of questions. Don't feel pressured to instruct a solicitor straight away.
Above it has been said that you may be placing yourself at risk if you sell then rent in the interim but want to argue that you need more than 50%, as you would have been able to adequately rehouse. If costs have been incurred by taking the matter to court and the judge considers it was not reasonable to do so in your case, you may even end up receiving less than if you'd accepted half now. Without knowing figures and details (ages of children, child
arrangements etc) and weighing up the costs versus what you might stand to gain it's difficult to say.
Your ex's admission he wants you to rent is helpful. I would be very wary of ending up renting and your ex continuing to sabotage the sale and delay the process for a very long time, even once you have agreed or a judge has ordered the distribution of the assets (court orders can be difficult to enforce - my children and I have been staying with family for 3 years due to my ex doing precisely this!). Of course, as you don't have a court order and he is not cooperating you also have this hurdle too (I applied to court to settle the finances 5 years ago!).
The right to reside in your home until the matter settles is a great advantage. He will eventually want his share of the equity (on that note, are you 100% sure when all assets are considered he would receive any at the moment?). As others have said, you will of course want to ensure you are not just continuing to grow his share with the mortgage repayments (keeping in mind the moving and renting temporarily will also be lost money, not to mention the loss of stability).
At this stage carry on having the estate agent chase the matter. As they are jointly instructed, both of you should be included in all communication. You are wise to keep all in writing and keep it polite and concise. For now you could email them copying him in explaining that you have tried to call several times and still not heard back from him but are keen to accept the offer, and hope that he will respond soon. Keep the updates and chasing in one email chain if you can.
It might be worth having a look at the TA forms you will be required to agree if the sale does proceed, as there is a lot of room there for the ex to delay and sabotage the sale too.
Keep listening for clues from him but don't reveal anything yourself. Sadly it seems he isn't going to play by the rules or be reasonable, and solicitors attempts to get him to do so in court will be costly and possibly even ineffective. It will probably help to work out what will make him feel he is getting what he wants, and also where you can afford to compromise.