Titan Oil Tanks

Not one for writing blogs I wanted to vent my frustration at the total idiots at Titan Oil tanks.
Story so far....
had tank installed 5 years ago
tank has 10 year guarentee
Tank gets large cracks in it
Titan engineer comes and takes a look and tells me that this year of manufacture is faulty and they have to recall most of them.
But...
The tank is single skinned and I need double !!!
Fine I think I will call them and pay the difference between bunded and not bunded
How simple would that be ....
NO they say you can have £300 off your new tank
but I say the tank costs £560 retail
and the new bunded tank costs £1000 so I pay you £440 that I think is fair
No they say not a hope £300 or nothing
so I have leaking oil tank no heating no cooking and have to pay £700
Your comments please