To try to keep media folders tidy I've set start nodes for different block's media pickers. Client has asked for more flexibility - so for it to default to the start node but allow them to choose any folder. I thought that if I set the start node but also checked "Ignore User Start Nodes" - then it would do this. I can't get this to work - am I completely wrong on the intended functionality or is this just a bug? Can't believe I've gone this many years without knowing what this field is for otherwise

That's not what the Ignore User Start Nodes does. For a user you can determine their start node, which means you can hide certain nodes from them. This is commonly done if you have multiple sites within one, then you can invite editors into just the one they need by setting their startnode to their sitenode. The ignore start nodes setting would then allow them to pick content in the picker from nodes that they normally cannot see

Well, in a way, that's exactly what I want them to do. Except it's a single site - single client. So we have a folder structure: /Gallery /Houses