Thanks!

Actually it was me who suggested the ex-LIS to the TA, having read all the details on this board ^ The end point of the 4th sector (i.e. whether to ZRH or back to VIE) doesn't seem to affect the BA website fare.

I do need to be in VIE - it would in fact have been even cheaper starting in ZRH (surprisingly). I don't really need the last LHR-ZRH part for that trip, but should be able to move it to a suitable date when there is more snow in Switzerland