Hi again.
Many moons ago it was mandatory for any existing non compliant tanks to be brought up to standards when installing a new boiler. This is now not the case, unless an IR (immediate risk), otherwise as Simon said you would just note it down on the paperwork.
You do still have a problem though. As I said above if the flue terminal is within 1.8m of the tank then no one should be installing it like that.
You have several options.
1. Install some sort of fire rated boundary as I mentioned above separating the two.
2. As SimonG said install a plume management kit which would safely disharge at least 1.8m away, which I purposely didn't mention until I spoke to an associate.
Or 3. I have spoke to an associate to clarify minimum distances for fire rated tanks. A triple skinned, metal fired rated tank for example requires flue terminals to be at least 100mm away, not the 1.8m plastic tanks require, double bunded or single.
Having said that building regulations now state that ANY new tank installation requires at least 300mm clearance on all sides for fuel delivery companies to inspect the tank. Be aware though that fire rated tanks cannot be certified by OFTEC and have to be signed off through your local building control.
At the moment these are your options, unless you reposition the tank of course.
If you decide on a fire rated tank I can happily pass the details of my associates company onto you.