I saw the patio misting system over on Uber Review which is cool but static (it gets hot at the end of my garden too, 4 days a year). If you need a mobile cooling system you can get a misting fan.

It boosts to reduce the temperature by up to 25 degrees and covers 950 square feet which is more than ample for my BBQ area and would probably cover the rest of my garden.

Available from Smart Home for just over $100, which in the UK would probably work out at $25 per day for how often we’d need it.