I need to fit a tumble dryer vent in a new extension but I can't find
anything that looks reasonable - any suggestions?

Buy a condensor dryer. Then you won't be fiddling around with incompatible
hole adaptors every time you replace it. It is also considerably less ugly
than a manky outlet blasting expensive electrically heated air dripping
mould onto the path. They also work, unlike all the vented dryers I've used,
which always kink their hoses when installed and shut down frequently.

Christian.