No contactable numbers available for Guaranteed Recoveries CC

I am a registered debt counsellor, and for the past two months, I have been battling to contact Guaranteed Recoveries. I require a Paid-Up letter from them in order to finalise a debt review and issue a Clearance Certificate. The contact number I have listed on the creditor's statement does not appear to be valid. If I get no response to this review, I will be forced to seek legal advice.