Alright, mostly to put this thread to bed, i've since learnt the intricacies of how these actually work and indeed they are machine made, i wouldnt recommend anything but deleting a field of them to fix a percieved problem due to the complexity.
A touch bit of advice for custom PW maps for anybody who wants to use it step by step.
Startpage - Flick on all PW settings in every box
Imported heightmaps go into elevation as bitmaps, then you pass along and generate all the stuff you need page to page after page, a heighmap island for instance might not have all the water, so adjust sea level etc. have a play with all the sliders and figure out what they do, reimport bitmap to start again. You can just draw using the tool else for from-scratch.
After modifying them all to how you want them, save the mapset, then exit and re-enter PW
On re-entry, get your worldgen file, go down to a entry to super-impose the changes on, then open mapset and turn PW buttons mentioned in first step off until they all read pre-set, check the screens (should be same as mapset) then export to your usual worldgen file or extract and copy paste.
Obviously full gratitude and courtesy to anybody you use a heightmap or the utility itself from. Common places to get them include other games like OTTD, Sim-City 4, or off the web. Where usually you can do a simple resizing job using common computer editing software like microsoft paint until they fit the dimensions of the image.