The 828E is a condensing combi according to the spec. Its sedbuk A, but
seems it doesn't modulate down below 13.4kw for central heating.
Ideally you should be aiming for 3 or 4 kw range otherwise. Also,
weather compensation has to be a must for a LPG system. I would also
move away from a combi when it comes to LPG, but other opinions may
differ.

You could always sell it on ebay and install something like an ATAG or
Viessmann boiler with the weather comp options. I doubt you'll get much
change out of =A33k(installed) and it sounds like you're leaning towards
oil anyway?