Very poor browsing today.

[#1]

How is browsing at the minute?
Can you open a command prompt and type 'pathping ntp.plus.net -q 50'

[#2]

Actually Chris, I think in my case it might, just might, be a radio interference problem. I've been using 802.11n for a while, but it's recently deteriorated, coincidentally at the same time that EE has introduced 4G coverage in my area. Do these operate on similar frequencies?
The high latency spikes have gone since I dropped back to 802.11g at lunchtime today.

[#3]

EE's 4G service operates on 1800MHz (1.8GHz). And 802.11n operates on 2.4GHz and 5GHz (if you have a router that can use the 5GHz band - the TG582n only uses the 2.4GHz band, I think). But 802.11g also uses the 2.4GHz band.
If you can get things working on g but not n, then it does sound rather like an interference problem (the lower throughput of g would make it a bit less susceptible). Have you tried using different channels, and using a tool like inSIDDer to check which channels are free (or less utilised) in your location?

[#4]

inSSIDer shows only channel 6 is not regularly occupied, so I've changed to that, but even when using 11n it didn't make any difference. The only other thing I've done is reduced the bandwidth from 20/40MHz to 20MHz. Apparently some wireless adaptors are happier at 20MHz.
The biggest contribution to stability is just using 11g.

[#5]

Thanks for the replies nozzer interesting to read how you resolved that.