However, traceroutes to affected sites ( ebay.co.uk for example ) are showing 80 - 90% packet loss at either of two plusnet nodes....
Clearly not within my premises but within plusnets internal network.
Responding to ICMP packets is optional. It may be that those routers are very busy and are just prioritising more important traffic. That would still suggest a busy network but not necessarily a fault. If there actually is a fault on PN's network I'd expect a lot more complaints but time will tell on that