Hi Tabatha,
Having no luck with that boiler/installer.
Just so you know. you have a warranty on the boiler. If the boiler has a problem the boiler maker will send someone out. If the boiler is ok but there is a fault with installation. You will get billed for the service call out.
Your installer should stand over his installation for 1 year.
On the pressure thing, I have my doubts about leak sealer but have recently purchased a bottle for a Swedish crowd that is meant to be very good. I will try it and let you know how I get on. It is quite expensive though compared to other sealers on the market.