It sounds plausible enough for a company to offer system owners the opportunity for an expert to come in and give reassurance that everything solar is ticking along safely and performing optimally but things aren't always as they seem...
On safety...
What exactly is a PV health check?
There is no clearly defined standard of what is involved in a "PV health check" and nothing to ensure that the person doing the checking has the requisit knowledge, training and experience for it to mean anything. The PV health check is not a recognised standard or proceedure. It is of the company’s own design, and as such, does not offer any meaningful assurances of performance of safety unless you trust the company to begin with. The only kind of health check I offer for domestic customers is a PV EICR. This is a British Standard Electrical Installation Condition Report for Photovoltaic solar installations. This is a recognised standardised assessment report which can only be performed by a qualified, competent electrician. An electrician is liable for the their reports and should alway carry professional indemnity insurance to cover that liability. If safety is genuinely a concern, only a registered competent professional and a report that meets recognised British Standards really cuts the mustard.
On performance...
It's easy to check your own system performance by taking regular weekly or monthly readings.
If you’re simply never going to be that person or can't easily access the meter, I recommend that you consider system monitoring for your solar. Noticing an outage quickly, or being made aware of smaller, longer term underperformance can both save you significant cash in lost tariff payments and electricity savings. It can be a difficult for even competent and well-equipped solar engineer to verify system performance on a cloudy day. Arguably, your generation meter does a more reliable job if you know what to look for. Doing this will help you to confidently spot problems in short order without the need to pay anybody for the priviledge. See my article here for a guide to meter monitoring.
The truth about solar PV health checks
Because my business is focused on solar maintenance and repair I get to speak to a good number of PV system owners. At the beginning of June 2020 I was called for a price to replace a failed inverter following a visit to my customer from a “free PV health check company” who claimed to have taken over the customer portfolio of their original installer. During the visit they discovered that the inverter wasn’t working. This company quoted £2800 for what was a £740 job. If the customers son hadn't resisted the there and then sales strategy his mum would have been royally ripped off. This story is really only the tip of an unscrupulous iceberg of mis-selling and ruthless profiteering in the green energy sector, and is sadly, much more common than one would like imagine. If you've been sold a voltage optimisation unit (VO) for domestic use, a solar edge upgrade, or a home battery, on the basis of 30 percent energy savings, 25% increased solar generation, or payback on investment in less 5 years respectively, you may have been mis-sold and not yet know it. The experience of a small but significant number of my customers is that PV health checks are commonly offered by unscrupulous companies as a method of getting a sales person into your house. Once there, a combination of high-pressure sales tactics, objection handling and technical jargon is employed to part you with your money. I would implore you to only let these people into your home with a view to filming them and submitting the evidence to strading standards.
If you get a cold call or a letter from a company you don't know but who seem to know you, you should be suspicious and not reassured. If you feel you've vastly overpayed for a service, that you've been mis-sold or ripped off please report it to trading standards. Please shout about it in online reviews on every review platform you can find. When you leave a review, keep to the facts. Though you may be justifiably furious at the way you've been treated, keeping it clear, and free from expletives will lend credibility to your voice and help to prevent your review being removed from sites like trustpilot.
https://www.gov.uk/find-local-trading-standards-office