I got hold of a 20 A Altai choke, which has a shielded core and separate ground wire (for bodyshell earthing). It's pretty meaty, cost about Fl 25- (GBP 8.00). It has done a truly excellent job of line conditioning. Since Altai are available just about everywhere, if anyone's interested I can get the Altai part number and post it here.

One of the few remaining Mk1 owners... #00015