www.ultimatehandyman.co.uk

Never had an issue i have 2 runs of 25mm down the garden for 10 plus years joints glued and clamps every meter or so, the threads in that box are standard size and designed for conduit and gland fittings the fittings you have just remove the lock ring and screw it into the box, either way, you will still get some moisture to build up over time unless you want to be really anal and put some desiccant bags in the box

as for the corners you can use inspection 90's or round angle boxes

I spoke to the guy at Edmundson yesterday. Apparently there isn't a complete waterproof fitting that attaches over a 20mm conduit pipe. He recommended either I use solvent weld as I first thought or have the pipe enter the side of the waterproof casing and create a drip loop. If I choose the second option I really don't need the waterproof case. I can just connect directly to the light.

I'm surprised there isn't an ip version that fits over conduit pipe.

if its a cable coming out of the wall just use a round end stop box, a female adaptor and a gland with a snag end of conduit that will give you a flex outlet

Use solvent weld and dress the cables so that if water gets in they form a drip loop before your electrical connection and fit a drain hole in the box