I have tried this and it has been good for 20 mins. Other users are saying it is temporary so will check back in again tomorrow. Thanks for the suggestion.This seems to be the solution: Turn off Unlock with Apple Watch.
See this thread https://forums.macrumors.com/thread...es.2220783/page-2?post=29381330#post-29381330
I did this yesterday and so far, I haven't had any more 15mbps dips. Fingers crossed it's fixed.
I have a Google Home Wifi which doesn't allow end-users to change the frequency range or the channel. From reading that thread, the issue is worst with 80mhz width channels and channel 149 - which is what Google thinks I should be at.