Thank you both for your replies.
I've had a look at the logs but there nothing that really jumps out at me, probably because I don't really know what I'm looking at.
One thing I have noticed is that whenever I wake up my office PC it causes a bought of latency - I tested this by running a ping from Fing on my phone, it was fine until my PC woke up.
This corresponds with the logs below at 16:28 and 17:50
The Gateway often does report latency, or that it's down - I've disabled gateway monitoring to see if that has any effect. I thought I had previously told it not to monitor 8.8.4.4, but it seems I probably didn't apply the changes or something.
Gateways
Apr 14 17:50:50 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Clear latency 181769us stddev 200885us loss 16%
Apr 14 17:50:20 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Alarm latency 251851us stddev 213510us loss 21%
Apr 14 16:28:21 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Clear latency 93324us stddev 148846us loss 12%
Apr 14 16:27:32 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Alarm latency 123820us stddev 149520us loss 21%
Apr 14 12:53:33 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Clear latency 205517us stddev 243997us loss 15%
Apr 14 12:53:01 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Alarm latency 274140us stddev 246650us loss 21%
Apr 14 12:23:36 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Clear latency 287337us stddev 249855us loss 16%
Apr 14 12:23:15 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Alarm latency 340286us stddev 228988us loss 21%
Apr 14 09:33:14 dpinger VIRGINMEDIA_DHCP 8.8.4.4: Clear latency 249639us stddev 219601us loss 19%
It also had the same effect when starting a different PC, but no entries in the above log.
'Register DHCP leases in the DNS Resolver' was checked so I have unchecked this for the moment as well.
It doesn't look like DNS rolver restarts are my problem, as there's nothing list after 10:22 this morning.
DNS Resolver Log Entries
Apr 13 10:22:34 unbound 87991:0 notice: Restart of unbound 1.9.1.
Apr 13 10:22:34 unbound 87991:0 info: server stats for thread 3: requestlist max 0 avg 0 exceeded 0 jostled 0
Apr 13 10:22:34 unbound 87991:0 info: server stats for thread 3: 0 queries, 0 answers from cache, 0 recursions, 0 prefetch, 0 rejected by ip ratelimiting
Apr 13 10:22:34 unbound 87991:0 info: server stats for thread 2: requestlist max 2 avg 1.33333 exceeded 0 jostled 0
Apr 13 10:22:34 unbound 87991:0 info: server stats for thread 2: 3 queries, 0 answers from cache, 3 recursions, 0 prefetch, 0 rejected by ip ratelimiting
Apr 13 10:22:34 unbound 87991:0 info: server stats for thread 1: requestlist max 0 avg 0 exceeded 0 jostled 0
Apr 13 10:22:34 unbound 87991:0 info: server stats for thread 1: 1 queries, 0 answers from cache, 1 recursions, 0 prefetch, 0 rejected by ip ratelimiting
Apr 13 10:22:34 unbound 87991:0 info: server stats for thread 0: requestlist max 19 avg 13.82 exceeded 0 jostled 0
Apr 13 10:22:34 unbound 87991:0 info: server stats for thread 0: 100 queries, 0 answers from cache, 100 recursions, 0 prefetch, 0 rejected by ip ratelimiting
Apr 13 10:22:34 unbound 87991:0 info: service stopped (unbound 1.9.1).
Apr 13 10:22:34 unbound 87991:0 info: start of service (unbound 1.9.1).
Apr 13 10:22:34 unbound 87991:0 notice: init module 1: iterator
Apr 13 10:22:34 unbound 87991:0 notice: init module 0: validator
The pppoe0 interface errors look a little odd, as that interface is disabled so why is it trying to get an IP address?
Routing Log Entries
Apr 14 18:58:00 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 14 18:43:29 miniupnpd 46112 upnpevents_processfds: 0x801c15100, remove subscriber uuid:d55e1653-7e7f-11ea-9d08-004243ad0314 after an ERROR cb: http://192.168.0.11:2869/upnp/eventing/rvptmhmbsp
Apr 14 18:43:29 miniupnpd 46112 upnp_event_recv: recv(): Connection reset by peer
Apr 14 18:43:29 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 14 16:36:18 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 14 16:27:19 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 14 12:06:12 miniupnpd 46112 SoapMethod: Unknown: GetPortMappingNumberOfEntries urn:schemas-upnp-org:service:WANIPConnection:1
Apr 14 12:06:12 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 14 04:57:19 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 14 04:47:16 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 14 04:31:15 miniupnpd 46112 upnpevents_processfds: 0x801c15100, remove subscriber uuid:c74182b5-7e08-11ea-9d08-004243ad0314 after an ERROR cb: http://192.168.0.11:2869/upnp/eventing/ckkmjuoyes
Apr 14 04:31:15 miniupnpd 46112 upnp_event_recv: recv(): Connection reset by peer
Apr 14 04:31:15 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 13 20:45:42 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 13 20:36:42 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 13 20:18:12 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 13 20:08:41 miniupnpd 46112 Failed to get ip address for interface pppoe0
Apr 13 20:00:11 miniupnpd 46112 upnpevents_processfds: 0x801c15080, remove subscriber uuid:61ec7033-7dc1-11ea-9d08-004243ad0314 after an ERROR cb: http://192.168.0.11:2869/upnp/eventing/rqpxpugbah
I'll leave it at that for tonight and see if anything improves, or if anyone has anything more specific let me know, although I probably won't get chance to look until tomorrow night.