I experienced the stress (and trauma) of this. Went from open to closed system but oil boiler (Grant) was showing pressure loss. After running away with the money, the plumbing team said "the loss is significant" but hey - never returned to help solve the issue. Hired another firm who drained and filled the system with argon (?) gas. Leaked water through a busted joint onto my living room carpet. Turned out they did not know what they were doing so buggered them off and took the hit. For a few years was worried had a major leak in a corroded pipe in the living room. Trades are great at scare stories.
Had boiler serviced every year by local guy. Told me "the boiler is an Audi" and "all is good."
One night I went out when boiler was on wearing a head torch. I could see wet ground around the boiler. Put my hand underneath. It was split apart. Outside boiler fitted and nowhere near enough insulation to protect it from harsh UK Winter really. Neighbour up the road just suffered exact same thing. Brand new boiler split from frost after less than two years.
I have a Biomass boiler now. Sits indoors. Can service it myself.
My advice to LCC is to make sure that more than one person looks at the boiler. And also put kitchen towels at every rad vavle as someone else said.