huh?
you saved £400 on your install, but if there's an issue and it doesn't work how much of that £400 could potentially be spent trying to fix the problem? even if they fix it for free, you have to travel to and from the installer which is petrol money and your time. i value my time at around £40 an hour because that's what i could get paid doing overtime work, so a few trips to and from them trying to sort different issues and trying to help them find a solution and very quickly it might not be worth it anymore.