Your defunk installer, should have been HIES or had Insurance Backed Gurantee (IBG), as part of being MCS registered.
You should look on companies house, to see if the company has stopped trading or sprouted up under a new name. The MCS should help with any insurance they had. Proof the company went bump will also help.
Then it’s a waiting game till Gov Scotland respond, but any above information may help if they request it.
Ofgem or similar, will have the final say, but Citizens Advise may be helpful in the first instance and especially if you take it to court.