Negotiating on a new build - advice please

[Pweization]

So.......we have seen a (Bloor) house on a new estate in a nearby town. The house is one of the last 5 or 6 on the site, of maybe 100 homes and has just come back after its reservation fell through over lock down. There are a lot of new builds in the town, with more to come. We are very lucky that we do not have a mortgage on our current home, and have some savings, so for the right price, we wouldn’t need one. We are looking at the possibility of a PX - the full value of our house is £300k, and the new build Is up for £400k. We would need to make the difference between the 2 (Ie additional money we would need to pay) closer to £50k Question is, what sort of offer are the builders likely to take?
In our favour: it’s a stock house And I’m guessing if we moved quickly we could complete by the end of their FY (end June)
Against us: the unknown market, the price difference.
Is there anyone who works for a house builder, or who has had recent experience willing to give me a realistic expectation? What’s the best anyone else had negotiated?
Thanks

We bought our David Wilson home in March last year - it was one of the last 5 and they wanted it gone by end of June.

It was initially on at 450k, we got them down to 430k and they paid our stamp duty, plus threw in 25k worth of upgrades and flooring.

We’ve since become friends with the guys who sold us the house, and they were convinced we didn’t want it.

We were in a v good position of being FTB with a sizeable deposit and on a rolling rental contract, so could move in within three months.

Update - we heard back from Bloor on Weds evening. They have valued our current house at £300k, which is bang on what we thought. But they are not willing to look at PX, or even allow us to reserve a house is we went down the assisted move route with them. We don’t want to do this. It seems that maybe they are more confident of the market than I thought they would be. So that’s us out. Never mind, onwards and upwards. It will make a lovely home for someone. I’ll just grimace every time I pass that house 😝 And keep an eye on the market for the future.

If you can sell your house first that's where you'll get your negotiating power, but in my experience the negotiation comes with incentives more than house prices, they don't like to negotiate the price too much as it impacts on the rest of the houses they have to sell. When we looked at PX we wouldn't have got barely anything, now we have a buyer we've got £8000 in kitchen upgrades and flooring, £8500 stamp duty paid and £3000 cash back. We are having to go into rented between sale and purchase but thankfully that's easy for us.

So if you can get a buyer you'd be in a really strong position for a new build, especially if it's built, although you obviously can't pick things (we couldn't, it was too late).