I've had Virgin Broadband for a good long while now and for most of that time the service has been excellent. Over the last few months however I've found that speeds have been very low in the evenings - any time after 8 so far as I can tell.
Along with very slow download speeds there is a ping of up to 500ms to my local speedtest server.
I've found that sometimes running the diagnostic on the website causes the service to improve but only for that day so it's not a proper solution.
Really sorry to hear you're experiencing speed issues.
I've located your account and I can see an issue with the network in your area affecting speeds at peak times - Ref.(F003733824). Our technicians are working to get this resolved and have set a review date of 29th November 2017.
I'm really sorry for any inconvenience this may cause.
All the best
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