It looks really great, so clean and simple. I have shared your photos with my husband and those along with the easy to keep clean appears to have swung it in favour of boarding. (interesting as I'm the one who cleans the bathroom :) ) I will get some samples and then we can finally make progress thanks to everyone for your help.

There are a few guides online, e.g.  https://www.mydailyrenovation.co.uk/diy-wall-panelling/

By inlets, do you mean the recesses? I've just had Multipanel panels installed in my new bathroom. External corner trim was used around the window, but a piece of solid surface worktop used for the windowsill.

Thanks for the info and link, Sonia, much appreciated :-) They are very strange aren't they! But the house may be just that much older than the usual Victorian houses I've seen, so maybe these are Arts & Crafts mouldings. I just think the mouldings and some of the dadorails are so thin that they aren't in keeping with the size of the rooms the way the cornices and skirting boards are, and maybe that is why they are additions. Thanks again!