Hello sorry to here your troubles, 400 to clean the sensor is a bit steep, a new one should cost that much in the 1st place, best trying to source parts outside the dealer network www.importcarparts.co.uk is good and fairly close to you.
There seems to be a lot of differing info regarding the code, normally the downstream sensor located behind the cat is the culprit,
Have a look here for a bit more info
http://www.subaruforester.org/vbulletin/f66/po420-cel-code-catalyst-system-efficiency-below-threshold-26140/#/forumsite/20556/topics/26140