They have doubled checked the boiler, found no leak except a very
small drip, they have now put a sealent into the system to stop any
leaks.

Hmm, quotes like that make me very nervous and suspicious about what I
am being told. A "very small drip" is quite enough to cause the
pressure to drop substantially. 0 pressure does not mean the system is
empty, is simply means there is insufficient water in the system for
it to be under pressure. If you have a small leak, if you pressurise
the water, that leak will be a *little* worse.

If the small drip is occurring on pipe work installed by the company
who fitted the boiler, the work IS substandard and completely
unacceptable. They should fix it, now, and for free.

It is possible a leak has developed elsewhere in the system, even
though your previous system retained pressure. But I would say it is
unlikely. Therefore I think adding a 'leak stopper' to the system is a
bit of a con, to fool you into thinking they are being helpful. Try to
identify any water (no matter how little) occurring on pipe work that
they have installed; this sounds like the cause.

Regards commissioning, it is very difficult to advise since I do not
know what you agreed before hand. Having said that, 'commissioning' is
the bit that is signed off (and your boiler warranty likely void
without it); therefore even if it was not agreed beforehand I think it
reasonable to assume this would be included in the installation
costs.

Don't let them get away with it.