//…but I've noticed things slipping in the last week - e.g. I've seen a lot more family visits/overnight stays at neighbours , teenagers gathering in groups again and people going shopping twice a day for something to do,//
Indeed. Take a look at this thread:
https://www.theanswerbank.co.uk/ChatterBank/Question1704416-4.html#answer-12400388
The usefulness of the lockdown will reach a natural conclusion by about the end of May. The “breakouts” that you describe will become more widespread and those who choose to will simply ignore it. No amount of mathematical modelling will alter that.
//…there's a risk things may not fall as much as hoped for unless we have a short strong tightening to shift things downwards more quickly.//
And how, pray, will that be achieved? It is not possible to enforce the current regulations, let alone anything more severe. Where observance is taking place it is happening by consent not because of enforcement. It is simply not possible to enforce any sort of lockdown without the public’s consent and that is very likely to be withdrawn in large quantities soon. Yes, there are people who support the lockdown and they may continue to do so. But there are lots who do not.
//Sweden's model at least partly benefits from lower population, lower population density, and so on, and as the number of deaths per million there is still growing it's not clear that they have really done better anyway.//
Sweden may not have done any better (though that’s yet to be determined) but they certainly haven’t done any worse. The UK currently has a little over eight times the number of hospitalised infections and more than nine times the number of deaths though it only has around six times the population. Of course, as you mention, population density must play a big part but one thing Sweden has not done is to virtually paralyse its economy to the point of trashing it and has taken comparatively very few lockdown measures.