About a month ago I built a discrete-time version of a SEIR model, because it seemed to me that there were far too few deaths for covid19 to be worth worrying about.

I picked what seemed to be sensible parameters - and lo, when ICL released their latest model, it turned out I had picked very nearly the same values as they had (I had a longer 'infectious' period).

The model is horribly unrealistic on a bunch of levels; first, there's no genuine constraint on the number of infected - such constraint as there is, happens as a result of a stupidly unrealistic mechanism for the transition equations for 'Susceptible'.

I replaced the stupid mechanism with one that was based on behaviour (i.e., the number of interactions that infected people have per day, and the probability that an interaction infects someone else).