Internet speed has dropped significantly.

[#1]

Sync was no better with the DECT not attached so I don't think you can blame that.
As the sync speeds are pretty consistent no matter what has been tried to looks almost certain to be a problem between the master socket and the exchange.
You need to raise a broadband speed fault if you have not already done so: http://faults.plus.net

[#2]

Or a damaged master socket, or a faulty router cable - try swapping them (you must have one with each router)

[#3]

I've tried the router with 3 different cables all with no sucess, the master phone socket looks fine and looks like there is nothing wrong with it.
Thanks for the advice ive just raised a support ticket, and thanks all for your replys your help although unsucessful has been very useful.
adie:quote