SurreyGirl - this is a common problem that we find nowadays in so much as people that used to rely on competent and reliable tradesmen to source 'specialist' parts can now find them with a few internet searches and then assume that they're being ripped off. I'm both a competent electrician as well as a competent businessman. I therefore make no apologies for reserving the right to add a few quid onto a significant component where appropriate, as I have gone through the process of sourcing, 'bank rolling', accounting, bank charges etc etc on your behalf. In return, you get the right to come and knock on my door if it goes wrong. If you want to save yourself that few quid then please be my guest, I'll still come and install it for you, but if in two years time the supplier has gone bust and you're in a pickle with a broken system then you'll get little sympathy! I actually suggest to some of my customers to go and source direct (and I'll even give them phone numbers and part numbers) as it makes warranty issues easier. NOT EVERYONE IS OUT TO RIP YOU OFF!! Though, there are some that are, admittedly.
As far as your papework is concerned - you should have been left with; 1) Installation Certificate [3 parts] by the installer, 2) any warranty information and 3) should receive at some point in a few weeks a Part P notification from a Part P scheme.
Item 2 can probably be resolved between you and the manufacturers if they're nice about it, but if 1 & 3 don't happen then I strongly suggest you report the installer to your local trading standards as they may be trying to pretend to be a Part P registered installer when not.