Thanks to everyone for all their help and advice! Really appreciated
I found in my local hobby shop some thin, narrow plastic strips that seemed to be the right width to cover the windows. So I applied this from the inside, and used some filler and sanded down from the outside.
To my horror, I noticed that this old kit (60s perhaps? - eBay purchase, paid way over the odds...) was missing a few parts. Thankfully it was only things like some cabin and cargo doors. I had already glued the fuselage together, but managed to poke in and glue some scraps of spruce, then that gave me a structure to support the filler.
I have to say though, that this Vallejo putty is pretty awful stuff. After several layers and sanding I just about got it to an acceptable standard, however - weeks later (I was waiting for a sunny day to do some spray painting on the balcony) the bloody stuff expands and reputtifies in heat/sunlight! What good is that to anyone!?