Back down to 4.6 mbps, this is a joke

The kids are now using our second broadband connection (Sky adsl) which only gives around 4.5 mbps but they now use all their portable devices on this instead as they reckon it's better, says it all really !!!!!
---------- Post added at 02:12 ---------- Previous post was at 01:40 ----------
Just rebooted shub2 and speeds suddenly back up to 38 Mbps which is the speed i've been experiencing throughout the day and early evening until it slows to a relative crawl anytime between 20.00 and midnight.....I can't fathom this, any ideas anyone ??

---------- Post added at 02:14 ---------- Previous post was at 02:12 ----------
Spoke too soon, within a few minutes it's back down to 4.6 Mbps.