Contradiction against contradiction that is what you are going to find when the powers that be interfere to much ,which is what is happening,the basic regulations are being stretched this way and that.
We had/have a problem about flues with extensions falling apart,instead of going back to the manufacturers and saying 'come on lads ,sort it out,improve the quality of the joints,they say 'have inspection points for concealed flues',so when they do fall apart,we can see(if it is a rented property or boiler starts playing up and ever inspected),make the flues ,so they can not fall apart.
You say we follow manufacturers instructions,in fact we can not,I fitted a vaillent with a 3.5m rum straight to out side,I drilled and screwed up joints but still had to fit twice as many wall brackets as mi said to make sure secure to my humble standards,if I had fitted brackets as they said it would have bowed at the joints,joints would have held as screwed together but excess pressure put on them and would look very poor but how often have we seen flue like this
I have purchased plumage kits and had to purchase and mess about getting an extra wall bracket as the one or two supplied not enough
Volkers used to be very popular boilers in the eighties and early nineties,now you see very few,(although making a come back)the sales dropped dramatically,not because of the boiler,which are quite good but because they brought out a cheap plastic flue that was larger than all others and the top elbows feel apart,we the fitters changed boilers for safety not the manufacturers and ,then corgi organisation,they all sat back
If the gas regulators took gas safety seriously at source ie,with manufacturers in the design stages,a lot of regulations would not require up dating because unsafe situations would not arise as due to the manufacturer quality they would not be able to... ,end of
And the costs involved in improving products are pennies,if done at the manufacturing stages
imho
