Why do I need additives?

To prevent cold flow problems, and microbial contamination. Others include lubricity for Low Sulphur Diesel, Conductivity, etc.

Why don't oil companies put additives in?

Cost. To meet minimum British Standards

Why do I need to dose at storage tank level?

So additives can protect both fuel storage and trains.

What is the process from here?

Contact FT, Decide which services you require and get a sample of fuel for testing.