Having waited a week for the modelling clay to dry, on closer examination I see my method of squashing and scraping with my thumb a large lump of modelling clay across and into the track has caused the sleepers to move and distort:
My how I laughed! Thankfully, this was done for only one-third of the layout. For the right hand sidings (general merchandise) I will make little 'sausages' and cut them off to push down into the sleeper gaps. For the passenger station viaduct, I will be using granite chippings and P.V.A. glue, so the problems will differ, no doubt. 'Let the shipwrecks of others' misfortunes be your lighthouses', or suchlike...