Thomarse wrote:

Does anyone know where I can get some basic hardboard, white, panel
effect doors (which in any other size you can pick up for £20 or so)
but in the following size...

Oh FFS, you're moaning about one hideous type of door installed by the
insane DIY fanatics of the 1970s only to consider replacing them with
hideous 1980s hardboard and paper crap.

Get some doors made by the local joiner, who can do a decent job. I've
had to replace an extremely non-standard door in a Georgian house and
I'm utterly grateful that we had the sense to get a proper one made
rather than to try and fit a recent pastiche.