I remember having to build a main supporting wall up through three floors of a listed building. Normally it would be subject to VAT. Because the wall had "migrated" about 100mm over the years, I had to put it back in its original place.
As it was now an alteration that (technically) needed Listed Building Consent, I applied for it.
Anything that requires Listed Consent is zero rated for VAT.
Planning Dept. thought I was mad, applying for Consent that I didn't need, but it saved the guy a couple of thousand!
So ............. new tank ..... re-site it slightly. Also, new rules require a lot more in the way of a soakaway system.......... covering more ground.
Put in an application. Once it's passed, a contractor should charge you zero VAT.
Worth the hassle :o)