At the time the cable was installed (72ish?) 230kV was a very high voltage cable. The type of cable that now commonly carries that voltage underground is HV XLPE

http://www.fujikura.com.sg/hv_xlpe.htm

At the the maximum voltage over XLPE was 110kV.

Sticking 2 of these cables side by side would have other interesting effects.

http://www.emfs.info/Sources+of+EMFs/Underground/multiple.ht...

It appears oil and paper systems are still used, but I don't think they use the high pressure oil system like the one in the paper.

http://www.hitachi-cable.com/about/publish/review/__icsFiles...