Connecting rainwater downpipe to twinwall pipe

[WWilts]

New build in progress.

Highly permeable site down to permeable bedrock.

Qu. How to connect 68 or 80mm downpipe to 63mm twinwall pipe?

[PeterW]

Twin wall being used for what ..? Connection to underground soakaways ..?

 

63mm OD twin wall is 50/51mm ID so you would have to use something like a spigot adapter however it will significantly reduce flow rates so it needs proper design. 
 

https://pbs-limited.co.uk/product/68mm-50mm-reducer-adaptor/

[Temp]

Why such small pipe? I'd worry about blockages due to leaf debris.

[WWilts]

Increased the pipe size