parkray chiltern to twin coiled cylinder help pls!!

HI TO ALL! I am a gas safe engineer and have to price up a new boiler job, british gas have also put a price in. the job requires a system boiler to a twin coiled cylinder the other coil to be connected to the exsisting parkray chiltern fire/back boiler this will require cutting of the heating circuit from the fire (apart from the one heat leak radiator)
parkray say it ain't a good idea but are they just covering there backs? the fire ain't gonna be completely sealed off after all and will be open vented from the cylinder circulators, its only the heating circs that will be capped off.
I just need help on this one cos i don't have a lot to do with solid fuel, thanks!

proberbly parkray are saying this is, due to the fact the heat ex on the boiler is designed for heating and dhw. hence volume of water heated by the fire may exceed the heat leaks requirement and cause overheating.