Found this thread while engineer completed BT modem swap. I was told that they were replacing all modems as they had faulty components and were overheating. Being the cynic I am, I suspected conspiratorial bugging of decent people's houses etc.... (well, ya never know!) So far, no improvement / deterioration in broadband speed. Pretty much same as before. Will wait to hear if other family members report anything.
BTW- it took 2 mins to replace.
North London girl