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A.

This is a tricky one.  Non-participation is always hard to measure in any engagement tool.  We don't have specific guidance around this but I do understand your point about understanding why they aren't participating. 

I would suggest one of the following (I'm sceptical about response rates because if they're not getting involved in PB its also likely they don't get involved in other ways):

1. Householder survey (within the area that PB covered) that could be promoted through local press or local council newspaper.  The downside to this is the relatively high cost compared to rate of return.  However, if another section of the council are doing a householder survey for other purposes you could request to add a few extra questions and so spread cost and also reduce consultation overload this way.  An online survey may reduce costs but you obviously need to consider the local population and whether they are more or less likely to respond to an online survey vs paper survey. 
   
2. Alternatively, the higher cost route but you're more likely to get more detailed and useful information (and a higher rate of return) route is to do door to door consultations.  A way of reducing costs here is to use volunteers for example from community groups to interview people that are trained in this (I recall some CENs do this for free/cheap) or students from a local college (related to courses to do with research or administration etc).  If I remember rightly, that if you use locally trained volunteers then the rate of involvement does increase because local people trust other local people more - not seen as officials or remote people. 
3. Utilising existing community networks, meetings, etc - either ask those involved in the networks/meetings if they participated and if not, could they fill out a survey or participate in a focus group.  This is more detailed and may give you different information as I would expect you will publicise the PB events through these groups and meetings so they're less likely to claim ignorance.  Also, if they're engaged in this way you could get specific data about why they're engaged in the groups and meetings but didn't engage with PB.  The downside is that the number of people you may get this way would be minimal if your promotion and publicity of PB to these groups/meetings was good to start with and may well be limited to those who couldn't attend on the day due to prior engagements or other commitments rather than a decision not to get involved with PB. 

Possibly a combination of the approaches - the householder survey/door to door consultation will give you more general feedback about the level of awareness and positive feeling of PB in the area and doing a focus group with those already engaged in other ways but not with PB could give more specific information about why people get involved with some engagement processes and not others. 

In terms of encouraging non participants to engage in this kind of evaluation - then some kind of incentive is always helpful.