Business Option 3 throttled?

[#1]

I've been running Plus Net Business Option 3 for about a year now and it seems reasonably bedded in to the extent where I set it up as the primary internet link on my dual-wan Vigor firewall.
Strangely enough I got some quite bad throttling/jerky video on last Sunday evening when I was watching Youtube video clips.
The PN link in theory yields 8.9/0.9 Mbs down/up.
After a while things were so annoying I set my Demon Business Internet link as the primary route (PN as 2ndary) and video clip viewing returned to normal. Both links are via the same exchange - Deptford and both use the BT infrastructure.
I pay the business premium price to avoid any throttling? Am I mistaken?

[#2]

Yes http://www.plus.net/support/broadband/speed_guide/download_speeds.shtml
It may have been limited to 2Mb/sec

[#3]

Thanks for the link.  My firewall stats were showing data rates lower than 2M at the time, but I guess the problem was related to the evening throttle.
I'll stick to Demon as the primary link, they don't seem to have the annoying constraints on their premium business service and are cheaper too.