Thistle Windows & Conservatories Ltd

I am having a problem with Thistle Windows in Aberdeen. I have with held the final payment on a contract ( work done over 3 years ago) until they fixed a water ingress issue. They are telling me they will not do the repair until I pay the retention sum. I have refused to pay until the work is completed properly. Their argument is that if we pay they can then issue the guarantee and then come back and rectify the fault. I cannot understand how this can be legal as in my view there is no guarantee system that would guarantee a known faulty product. On checking the Network Veka website I note that we should have been given a deposit protection certificate - which we weren't . I have also researched online and found a worrying number of complaints against this company. Also it would appear that the remove any complaints from their own website. Thistle have been back to inspect the problem and accept it is a botched job. The MD however will not allow them to fix it until I pay the £300 retention. I offered to put this cash in the hands of an Independant solicitor to be released to Thistle once the work was done to a satisfactory standard. they refused. Conclusion - they are not to be relied on and probably not worth risking your cash with them if they treat customers this way.