If you are a normal domestic property, provided your tank isn't bigger than 2500 litres, you probably didn't need to habe a bunded tank (you do now if you install new or change the tank). The Building Regs summary is here about the distances from water courses and perimater boundaries, plus minimum distances from buildings.
http://www.tank-repla....co.uk/questions.html
It won't come up in a mortgage lenders survey, but if a purchaser does a homebuyer's survey it should come up in the surveyor is worth his salt.
Why bother to put it right now? - wait and see if it comes up then negotiate a reduction in the price if you have to.