Very odd servicing options from Vaillant.

They do a fixed price repair for £311. This covers most parts, excluding
the expensive bits like the big circular heat exchanger and the
expansion vessel. (Why not the expansion vessel? It's not that
expensive, surely?)

Or for £312 they do 6 months cover from Domestic & General. This covers
any repairs needed during the 6 months, up to £1500 total, including the
initial repair.

It seems a no-brainer to go for the D&G option, then cancel after the 6
months. What am I missing?