Have you had an unsolicited letter from a company you've never of, offering a free health check for your solar PV system? My recommendation: bin it!
If you have had solar electric panels on your roof for several years then the inverter (usually in your loft-space) may well be out of warranty. Companies like BTSA (British Solar Trading Association) have somehow obtained this information and are offering a free health check. In all likelihood they will do a sales pitch on you to buy an extremely expensive extended warranty, or will try to sell you a new inverter. The best thing for you to do though, if you're concerned about your inverter's performance, is to get back in touch with your original installer or to look one up on the Microgeneration Certification Scheme (MCS) website. Unsolicited communications should always be treated with extreme suspicion, and these letters from the "BTSA" are no different.
According to Companies House, "B T Solar Ltd" was incorporated in March 2018 and is involved in environmental consulting activities. It has just a single director, and is registered at 39 Halton Brow, Runcorn WA7 2EH - which appears to be just an ordinary house on a residential street. It doesn't seem to stack up as a real association, does it?