Day2 PCR test on return to Scotland

Has anyone used a private company for the day 2 test on returning to Scotland?  The Scottish Government website says a portal by CTM must be used and the test purchased at a cost of £88pp (its also the same for returning to Wales).  Some private companies who are on the recognised list of the UK government and used for return to England are much cheaper but will the passenger locator form accept the code if its not the one specified by the Scottish Government?