Before going any further clarification: The trees are too large to fit inside a marquee (thanks Chris). Small trees or flower beds are no barrier to erecting a marquee and can be excellent features but remember you lose that floorspace in the marquee.
I also forgot to mention what time of year the event was planned for, more on that below
My suggested solution:
A 6x12m marquee is probably around the right size, you could fit a 6x14m marquee in and they could fit everyone in to a 6x10m marquee but 6x12m seems about right.
You could rotate the marquee 90 degrees and run it down the garden instead of across, I’d discuss this with Dave & Jenny and give them the option of either way. If they wanted to have drinks on the lawn beforehand then they could use the left hand side and run the marquee down lengthwise. Otherwise I’d run it across the garden as you can see more of the marquee, can fit more windows in and it generally makes for a more inviting marquee.
If it was a winter marquee then I’d bring the marquee as close to the house as possible and consider using a walkway across the patio to connect the two. In the summer I’d set it as far back as the tree will allow to make it look more impressive for guests on arrival.
The internal layout is very informal, the middle dance floor area can be used for people standing initially but becomes the natural focus when the music starts. Having the bar & buffet in the same marquee as the dance floor keeps the party in one place and chairs around the edge give somewhere for people to sit down if they really want to. Fairy lights in the ceiling, black and white dance floors, illuminated bars are all possible accessories.
This is a very very popular layout for all different size marquees. As you use larger structures the dance floor and bar area become larger and more flexible – you can add sofas and/or poseur tables to create a real night club atmosphere.
That would be my suggested solution at least.
Thanks for reading.