Well done Nick! Nothing to worry about in cribbing our format - we're novices at it too and have just evolved 'what works'...
I'll be in if final date is do-able! I can probably interest a few of our regulars too...
A few suggestions/thoughts that may help:
Dates: Look at tides for launch/recovery and river runs - some dates won't be suitable and you need to work around this. 3 days & Bank holidays suited us because of the distances and we felt more days made the journey more worthwhile for those further afield. A weekend may be better - certainly for the first one? To do well, it does require quite a lot of time investment and the days can be quite stressful trying to ensure everyone is ok and having a good experience! It was only after year three that I started to relax and I think John found exactly the same. We opted for May as the weather is warm enough for most, usually more settled than summer and easier/cheaper to get accommodation for those travelling to your event.
Insurance. After a fair bit of research, we were led to believe that this wasn't necessary. However, the registration form states that owners provide their own and are responsible for their own launch, fuel etc and the organisers accept no responsibility. We have basically just got everyone together and how much or little they participate is up to them - all we ask is they let us know if they want to go off and 'do their own thing' so that the rest are not left wondering/waiting. Obviously as a group we would (and have) assisted anyone in difficulty or broken down etc, but participants shouldn't look to feel safer and take risks that they wouldn't normally feel capable of - beyond that bit of reassurance knowing someone is close by. Most/all of those here are sensible, seasoned boaters so shouldn't be an issue, but we also did a poster campaign too and didn't want complete novices turning up to sea trial their eBay special they bought the week before...
Harbour Masters: Need to be informed of the event, format and approximate numbers - out of courtesy if nothing else. Also if numbers look significant, let HMCG know too.
Pontoon: This really helped our event as it provides a base/congregation point and easy access to shore /facilities (quite important if partners & kids etc are attending! It's also useful to be able to return to if anyone has problems or conditions turn sour. Also a good social opportunity to view/discuss boats etc. MSL may have a similar events pontoon in the area perhaps - or even harbour master?
Local info: Slipways, fees, parking, campsites, B&B' are frequently asked about. Other less asked stuff that's important to highlight includes sandbars, submerged rocks, ferries etc - not everyone has plotter or charts (I always have both, but others may have only used lakes or familiar waters)
Avoid anything competitive or 'race' orientated as risk increases and HM will then want insurance etc = minefield!

Fun events/skills challenges etc are ok, but do a risk analysis and think through what could go wrong and how you'd mitigate. Some simple do's and don't s for participants - like kill cords and life jackets is advisable, plus highlighting any local hazards that visitors wouldn't perhaps be aware of. I've always taken spare fuel in case anyone runs out, first aid kit, tools, thermal blankets, throw line, and a whole host of other 'essentials' like a bottle opener!

Thankfully only the bottle opener was needed, but you get the idea - not so much thinking for others as ensuring that you're not going to get bad press if something goes amiss - 'disaster averted during boating event' is a much better headline for the local rag than a tragedy. Plus everyone going away happy means they'll want to come again!
I really believe that these events work well as they are not only social (I've made some great friends - as have others) - but they encourage us all to get more out of our boating and experience new waters, look first hand at how other boats perform and what might be next on our wish list...
Above all, take lots of pictures! Everyone loves pictures/video of their boat in action.
Best regards,
Rob