Went into 50Cycles with the bike, soaked (had to push it all the way in, in the rain). The guy there was quite apologetic and helped get the chain fitted - the bike is up and running again.
...but the motor CLANKING sounds are identical with new cassette and chain. It's the motor. I did tell him this, again, and told him I'm going to want a replacement or a refund. He told me, "as you've done 2,500 miles I don't think you'll get that" and I replied that by law I am entitled to.
(this was an amicable conversation BTW - 50Cycles remained polite about it).
It sounds as though I'll probably have to have another new motor fitted soon. I'm quite divided over whether I should take this and give it one more chance, or demand the refund or exchange.
I just want a reliable bike, because I NEED that for my daily commute to and from work. I can't keep telling the company directors "sorry, I'll be late in this morning - bike going into 50Cycles", then "sorry, I need to leave early so I can pick up my bike from 50Cycles". My employers only have so much patience, and I as a consumer only have so much patience.
So there it is.