Thanks for the advice all
It's probably not worth the hassle and expense of getting a Broadcom router so I'll just leave it for now
I'm getting 40.02mbps on speedtest.net this morning and the connection passes my "real world" tests at least - I can stream 4K content from my Apple TV 4K box at a level where it's not easily discernable from a 4K Blu Ray (I did a side by side comparison with some titles I have on both formats). I had to really be standing up close to the TV to notice any extra compression etc on the ATV4K.
The only thing thats annoying me at the moment is i'm paying extra per month for 43mbit over 40mbit! Thinking I should just move down to a 40mbit service and save a few quid every month ... but the lunatic in me doesn't want to lose that extra 3mb