this is the thing, a reboot actually seems to cause the problem

did a bit of further testing on this.....

if i reboot my router either through the setup menu router/setup.htm or just by switching it off at the plug, then straight back on my download speed falls to almost unusable levels. This would explain the poor download speed after the power cut (power was only off for a minute if that)
If I leave the router unplugged for 10-15 mins the download speed returns to 6-7Mbps as normal

as I say the upload/ping is always fine

I can only assume that there's something wrong with the router