Final Update.
The drivers at http://www.wch.cn/downloads.php?name=pro&proid=178 work well.
In addition some nano's I had ordered arrived and they use the same chipset - the above drivers work with these as well.
FWIW, it appears a lot of the Chinese clone boards are using this chipset for the USB comms - suspect it is a cheap chip.
Also,
I ordered a genuine mega which arrived - and the original seller of the 'dodgy' mega had sent a new board plus a couple of packets of jumper leads to make me happier.
Got to stop this obsession - I now have.....
1 Uno (Genuine)
3 Nano's (as above)
3 Mega's - one real and two clones
1 Due
(and I confess I also have three itsy witsy tiny ones (plug direct into USB and only have a few pins) coming)
Stan