I’m no craftsman but I am a dab hand at flatpack. I want to build some type of den/climbing frame/swing in my garden. Are there any companies which make kits which can be customized/pick and mix? All help appreciated.

Dunster House worth a look. We’ve got one of their climbing frames, which comes flat packed. Lots of variation in options.

Delivered by a big truck and off you go.

Iirc service was excellent as is the product quality.

I just designed one on the back of an envelope with a mate, ordered some wood, and broke out the tools.
Two days later:
[url=https://flic.kr/p/4KBbNu]000_0099[/url] by Matt Robinson, on Flickr

matt +1
If you can build flat-pack, you can build play equipment
I used treated posts sunk into concrete at the corners, and made the rest up as I went along. The slide, steering wheel and telescope came from here

Two days later:

Teaching the kids what to do when it floods?

It was prophetic. Sheffield floods were later that year iirc.