Went back to working fairly soon after RobertoS posted.
I tried it from my PN connection and just got "Firefox can't establish a connection to the server at
www.groupon.co.uk." so checked with another ISP connection and all was fine. Got the IP address from the other browser so then tried a traceroute on PN :
C:\>tracert 65.53.232.213
Tracing route to 65.53.232.213 over a maximum of 30 hops
1 31 ms 51 ms 44 ms 192.168.1.254
2 55 ms 49 ms 50 ms lo0-central3.ptn-ag02.plus.net [195.166.128.207]
3 53 ms 46 ms 53 ms gi2-20-231.ptn-gw01.plus.net [84.92.5.153]
4 51 ms 49 ms 51 ms po4.ptn-gw02.plus.net [212.159.1.139]
5 46 ms 47 ms 50 ms igbtmdistc7503.msft.net [195.66.226.140]
6 49 ms 48 ms 48 ms ge-5-1-0-0.lts-64cb-1b.ntwk.msn.net [207.46.42.72]
7 53 ms 49 ms 48 ms ge-7-0-0-0.lts-64cb-1a.ntwk.msn.net [207.46.42.40]
8 129 ms 132 ms 127 ms so-7-1-0-0.ash-64cb-1b.ntwk.msn.net [213.199.144.158]
9 138 ms 135 ms 124 ms ge-5-2-0-0.bl2-64c-1b.ntwk.msn.net [207.46.46.247]
10 188 ms 189 ms 191 ms ge-5-0-0-0.co2-64c-1b.ntwk.msn.net [207.46.40.106]
11 203 ms 186 ms 199 ms ge-2-0-0-0.co1-64c-1b.ntwk.msn.net [207.46.43.178]
12 217 ms 190 ms 202 ms ge-0-1-0-0.wst-64cb-1b.ntwk.msn.net [207.46.43.185]
13 209 ms 204 ms 192 ms vlan57.wst-76e-2.ntwk.msn.net [207.46.46.38]
14 * * * Request timed out.
15 * * * Request timed out.
. . .
28 * * * Request timed out.
29 * * * Request timed out.
30 * * * Request timed out.
Trace complete.
Posted (but should have also included traceroute, and if I had been feeling better might have tried to get similar from my sister's machine via LogMeIn). Anyway she had an appointment at her dentist and some other plans and unfortunately by the time I saw a response she had shut down her laptop (rare, as it is often on all night!) so I could not check from her PN connection to the site.
Even after the third response was posted, for a short time there was still a problem...