Back boiler plumbing regulations help

Hi all.
As you probably have seen me posting in the threads on stoves, you can guess I'm in the market to get one.
I spoke to a local seller who recommended a plumber to look at our setup to check it out for fitting the stove.
Here's where it gets interesting...
Plumber arrived today and I showed him the fireplace with existing back boiler, then to the hot press....
'Woah', was his first word, followed by 'Hmmm'.
Not good...
He proceeded to tell me that the system was plumbed with no non-return valves and so the heating was very inefficient due to the oil heating the back-boiler, and the back-boiler heating the water outside in the oil boiler. The back boiler should easily heat all rads in our house hot, instead they only get lukewarm when only using the back-boiler.
Also, and more importantly, he told me that the back-boiler had 3/4" pipes running to and from it instead of the min 1" as required.
He said that this is illegal in the sense that it contravenes regulations, and is inherently dangerous as the pipes could potentially burst from the pressure if the fire gets too hot. It's an open system btw, just to clarify.

Before I act on this information, I want to get confirmation.
Does the information given make sense and Is it correct (ie. Is the 3/4" piping contrary to regs and how dangerous could it potentially be?)?.

Thanks for your help.

My OH has this opinion..

1. Non return valves should be fitted.
2. Good plumbing practice is 1" piping from the back boiler to both cylinder and heating system.(gravity circuit on cylinder and pumped on heating).

Basically your heating system seems to be inefficient with your current arrangement and your plumbers assessment is right.

Thanks Armada.
Could you ask your oh if he knows what the regulations are in relation to this?
I want to find out if it's just poor practise or actually contrary to regulations.

Thanks
Pique.

1inch unrestricted gravity primary circuit between the backboiler/stove and 3/4 inch heating circuit linked to oil boiler circuit(incorporating non return valves). Read my posts as i have many on the subject