Like you, I'm very dubious about such mailings but just occasionally you might find that they can lead you to something genuine. (I nearly threw away a letter about applying for a PPI refund for a credit card I used to have. It sat on top of my bookcase for several months until, just before throwing it in the bin, I decided to visit the website it mentioned and enter the claim number given in the letter. I instantly got a message saying 'Your claim is being processed' and, a few weeks later, I got a cheque for £3000!)
Take a look at this site and work through the calculator. It should tell you if there are any
genuine grants available for your property:
https://www.gov.uk/energy-grants-calculator