Lino on unlevel floorboards

Cover it with a sheet[s] of WPB (marine) plywood of a suitable thickness and screwed down at the edges and middle of the board[s] to enable it to be lifted if necessary.

NB: At a pinch, you could even do the same thing with hardboard!

BTW "plastering" over the boards is not generally a good idea for a number of reasons.

Cash

I tihnk green (water resistant) chipboard would be cheaper than ply. Hardboard has often been used for this sort of thing. Replacing the bad boards is also an option!

NT

Is the gap small enough that a few sheets of folded newspaper will fill it? Feather it out over a larger area with fewer sheets.

Oooh, now that's an idea.

Either lift the low boards and add packers on the joists to bring them up level, or screw 6 or 9mm WBP ply down all over.

I doubt I could get anything 'bob on' so if I was to start screwing stuff onto the boards I would probably end up with raised bits. As I said I am not bothered or I would replace the questionable boards or board over the lot (which would involve taking the bog out again, which is where I can't be arsed), my only concern is that I hope to sell the house in a few months and what doesn't bother me might bother a potential buyer. I wondered if there was some sort of glob which would set hard but not break up that I could spread over the offending areas.

Plaster with lashings of pva mixed in might do it

Flexible floor tile adhesive designed for wooden floors...

You mention a couple of boards being "low" but what about gaps between the boards...are there any cos they'll show up just as badly after a while of it being walked on . As others have suggested boarding the whole floor is probably the only solution.

Dear RDS Under such circumstances surely the appropriate course of action is to raise the two boards and plate the joists below with suitable shims of thin ply so the boards are now level.

But then he'd have to lift the WC