Sorry Mark. Just the Sunday or nothing for me.
I will be better prepared next year, I can plan around a two day event then.
Have you an idea of total costs?

I can only do one day Mark, and Sunday is much the better option, like Graham it's either Sunday or nothing for me.
I too would need to know what the cost is before committing. My pension only stretches so far.
Rules of engagement as you call them, probably a good idea that we formulate a standard set of what motorsport would call "Supplementary regulations", and have a briefing every event. I will work on a generic set which a few of us can have a look and agree, (or modify) then have them on our website.
Noise pollution is an open ended comment, I would be extremely disappointed to get there and for someone to say "too loud", particularly with only an opinion. I have run similar powered vehicles at the equivalent of county shows where there was show jumping, animal displays and such with no ill effects. I understand what you are saying, but it does need more clarification.
I've not seen a formal risk assessment. I used to do these as a part time job, for clients who were presenting things to the equivalent of Board of Trade. Whoever is doing it, just common sense and not overly worded is the best option.
FWIW, from the experiences I have had with these things both in NZ, in the US and the limited times here, every time you put the word "Race" or "Grand Prix" in the conversation barriers rise.
With my conversations for venues, I have yet to be declined when I say a group of us with these things , and I have photos with me, would like to come, gather together and play with them.
Just for interest I once spoke to the same person and said we would like to come and have a race. Instant decline. Even then changing tack and explaining more only softened the attitude.

There are db meter apps available for smartphones, which might suffice. Perhaps best to use more than one app, on more than one phone, and take the average to hopefully negate any errors.

While at Stretton I took the opportunity to use their sound monitoring equipment. Results from the 6 CKS running was that the loudest at 10 metre distance, with the background of a solid structure (ie no sound dissipation) was 37 dB.
Thought this might be useful. That one had an open ended exhaust , as the muffler had fallen off.

Oh in that case I might not weld my muffler back on 
Sounds better with it off and probably goes faster to