help needed with westfalia sink!

[eastwesty]

i'm trying to replace a leaking plughole and/or ubend in my 1985 westy club joker. i've managed to remove the bottom of the plastic ubend/airtrap beneath the plug hole, but can't remove the part that actually fits onto the bottom of the sink- the plughole itself just turns as i try to unscrew it...

anyone taken one of these out before? and any idea where i can get replacements?

thanks!