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Your ducting ^^^ would be 150mm internal. Slip it over a connector which would give you 150mm external.
https://www.screwfix.com/p/manrose-round-connector-white-150mm/145gy
user69121-
Thanks - how does this work exactly? I can't fit the ducting through the hole. So ducting from extractor to hole, into connector, then what, some other piping through the hole (I saw some hard PVC one in B&Q which is 150 external), then out to the exterior grill? Seems odd that the standard coring bit in that size range (152mm) isn't big enough to fit the standard size ducting ("150"/160)
And is there a better brand to look for? As I'm not in the 'get it today' rush I'm happy to order something from somewhere if it comes within a week or so - I saw this for example which seems more solid than the cheapo plastic thing that came with the Manrose kit - is there much benefit? https://www.toolstation.com/stainless-steel-bull-nose-vent-grill/p87963
CYOA
On the subject of ducting again - I picked up these despite reading reviews that though advertised as 150mm, they come out as 160mm. They do indeed come out at 160+mm.
https://www.screwfix.com/p/manrose-aluminium-laminated-flexible-ducting-hose-silver-10m-x-152mm/86676
https://www.screwfix.com/p/manrose-extractor-fan-wall-fixing-kit-150mm/12523
Are you just supposed to squish them into place? That doesn't seem correct but happy to be told otherwise before I return them. Where can I get actual 150mm ducting?