We had the exact same argument. He wrong-footed me at the start. The day after I told him I had seen the solicitor, a piece of card appeared on the kitchen table, with his estimated valuation, which was based on a Zoopla prediction 
So we got into a wrangle about how much it was worth. For some reason we just assumed it was based on our agreement. (In my defence, your honour, he was a major headfuck).
Fast forward 9 months. I had had to take him to court over the settlement. Judge told us to get a proper, official valuation of the marital home. It cost me and him £100 each.
Value came back exactly the same as the one I had had done a couple of months earlier, by an EA who I arranged (didn't even tell ex-husband I'd done it). Rather like @iwantmyownicecreamvan I met her outside the house, and explained the situation.
Unfortunately neither of our estimates turned out to be true. We had bought at the top of the market. But in this case, there was no upside to a low valuation really. We had lost over £60,000 on the house, so the marital pot was reduced. Less for everyone, but Oh the Freedom.