Do you own these floors or are they rented?

Sue

It makes a difference in what I might suggest.

But, on the other hand, your response has just made me bow out.  Good luck on whatever you do.  I hope it turns out all right.

Sue

Hi . . . .
I've done this myself and it's fine.  If your property is rented, just check what modifications you're allowed to make but I really don't think there would be any problem.
The hard mash would be good.  Keep it as dry as possible (this helps drying time and also minimises shrinkage) and force it into any gaps with a hammer and bolster chisel (or similar).  Of course we assume there is no problem with dampness or spillage but you can always give it a coat of varnish when it is thoroughly dry.
If the gaps are quite large you can use a base filler, dependent on the size of the gap.  If it's large, pieces of wood with a bit of glue.  If not large, screw up some paper, perhaps with a smear of glue/paste and force it into the gap.  You can then push the mash on top and smooth it off with the flat blade of a knife (or similar).
If it is a bare floor, mix a bit of pigment into the mash to match the floorboard colour as closely as possible.
Have fun and be prepared for slow drying but then a lovely hard finish. 

David Osborne

You little beauty!  Thanks for that advice. This old house of mine is about c 1875 and like many old Sydney terraces, it's been damaged throughout - I'm getting hooked on scraping paint and now for something different - a bit of paper mache.  Just one other thing - is newspaper just as good as anything?
David