Spoke to HM, very helpful chap.
Basically you need to be launched and out of harbour by about 11am at the latest as there is a bank which has built up at the entrance. Should be about 1m at LAT but it is currently about 0m at LAT. Its not shown on the charts. Its spring tides that weekend so tidal heights will drop quickly.
There will be no charge for launching if you mention your all together and going to the Scilly's
Although under 6m is usually free anyway.
Parking for car and trailer available in the car park. Will need two tickets. He's not sure whether you can put enough on for the three days but has said if theres an issue to contact him and he will sort something out.