I cannot see why there'd be a leak on the joint in the first place
if it's not been dismantled. The only thing I imagine to cause this
is corrosion in the caliper as the seat of the seal is inset on one
face and the mating opposite is firmly mounted against this.
Could it not be inconceivable that corrosion has actually eaten
through the bolt and it snapped or corrosion on the mating face
to the alloy arm has leaned the caliper and snapped the bolt?
Drastic but why else would it leak unless it has started to fail
due to corrosion eating in somehow?
Another verification of common suitability is the GSF listing here:
http://www.ebay.co.uk/itm/Rear-left-and ... 570wt_1195
The plan to borrow the Activa one will work but couldn't it
be easier to get one off a breaker Xantia tomorrow just in
case you snap a bolt on the Xant!? Been here going backwards
like this with jobs in the past robbing Peter to pay Paul without
fully addressing the whole job!