Do not get a DG834 if you are going to stay on the C&W service. C&W equipment in the exchange performs poorly with the DG834 and C&W will simply say that the router doesn't work well with their kit and they wont care.
Your low connection speed both before and after the switch indicates that your line should never have been moved to an ADSL2+ service - generally speaking connections slower than 3.5Mb/s will not benefit from ADSL2+ and some will see reduced speeds as you have. As others have indicated C&W's insistence upon a 9dB profile means your speeds will be even lower.
The solution to this is to get away from C&W. Ask ADSL24 if they can offer you any other LLU network. Usually, but not always, the O2/BE LLU network is available where C&W is and that provides a lot more direct control.