Contact the tps (telephone preference service) again via their complaints department - here's the link
http://www.tpsonline.org.uk/cgi-bin/tps_compla ints.pl
and register this company there. You might also wish to contact the enforcement agency (information commissioner's office) and let them know that this company are still pestering you.
www.informationcommissioner.gov.uk
I used to get a lot of unwanted sales calls and until I registered with tps, I used to simply answer the phone, telling them they would have to speak to the master of the household, then I'd put the phone handset on the top of the tv, turn said tv up really loud and leave it there. A different way of getting rid of them was to be pretend to be very aged and exceedingly deaf (no disrespect to anyone of advanced years or with hearing problems) - that was actually quite fun !