On Sun, 1 Sep 2013 05:26:16 -0700 (PDT), wrote:

I may have to reverse this in order to make our replacement single skin
oil tank Oftec compliant (thus avoiding the cost of a bunded tank).

Surprised you can get away with a chnage of oil tank without it
having to be bunded these days. Or is the drain you are trying to
seal affecting a bund that is not part of the tank?

Will there be a suitable fitting to join (in an oil tight manner to the
cut off bit thats buried in the ground?

You can get "drop in" collars. One of those well silicone sealed or
possibly solvent welded(*) in place should do. Might have to replace
more of the down pipe so you don't have any more joins than
absolutely required below the prospective oil level in the bund
should a full tank leak.

(*) Check plastic types and suitablity of sealing goop to make a good
reliable joint. I have a sneaky feeling that silicone won't bond to
"down pipe plastic" in any lasting manner, but not sure that "down
pipe plastic" is solvent weldable...

--
Cheers
Dave.