They may just be doing some network upgrades - if so it usually only happens for 2-3 nights between 12-2am.
If there is a known issue however, it will be reported on the “free & automated” Service Status number - 0800 561 0061 - which usually gives the most up to date info. and tells you of more local issues down to postcode level.
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John
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I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router +2 Airport Express's. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.