@Pengy Without meaning to seem cynical choose your installer wisely!
An installer that is after max profit for minimal effort will glue them on for you typically using flimsy plastic or tiny little metal brackets. Perhaps if you are lucky he will add a few self tappers for good measure and you can then drive off into the sunset with fingers crossed.
There's a couple of people offering what appear to be ancient inefficient domestic panel installs on eBay at the moment £400-£500.
That includes about £75-£100 worth of parts, often including those awful cheap
PWM solar controller that go for about £3 in the trade!
I bet none of them have public liability or indemnity
insurance if their install fails!
Solar panel installation seems to be the new Wild West at the moment there clearly some profit to be made by using the cheapest possible components.
Good luck.
I chose the safer option, i.e to bolt mine on and that had the added advantage that I could do that anytime of the year.
There is a really big advantage to my preferred fixing method, that of flexibility.
If I want to change my panels of move them about or build a tilt frame I can with ease .
I use Unistrut as the mounting system.
The flexibility means you can experiment as well albeit with a soon to be replaced way too big panel