I have put in a pair of Digisenders. I got them off e-bay for £40 but they are sold in Agros (
http://www.hard2buy4.co.uk/i_742.html) . In order to use you remote control unit, you need a Gold version, but there is lots of information about this and for me that was not an issue.
Here is a simple overview. They are very simple to use.
http://www.geocities.com/digitalsatuk/digisender.html
I hope that this helps.