Thanks. Fewer than 10 plots in total on this site, they've done a few nearby, so we know the general standards. Planning permission has been given for the whole site, but we could apply for changes it if we wanted. The ones currently in process are being done as this model (developer might not be a totally accurate term here, tbh).
I need to check on services - whether we can come in as part of the overall site connection, or would need to do separately - didn't occur to me until after our discussion.
I think the trade-offs you mention are why they're deciding how they want to sell the plots - I've spent a couple of hours talking to the main architect, on site and seen the current in process builds, and think they're genuine that if they go with the option of helping us construct the shell, they'll properly commit. If they decide to not do that, it will be plot only. Obviously we could go in plot only up front, but that would be committing to building it all ourselves. We're not ruling it out, but there are other things that could impact on the time we could dedicate.
I think I understand the potential problems people are raising, and they're definitely worth considering.