Thanks everyone for your assistance and ideas. I have now made the following booking:

LHR-WAW-BUD (LO)
BUD-ZRH (waitlist on both LX and MA services)
ZRH-IST (LX)
IST-ZRH (TK)

I was advised that transiting ZRH twice would make the IST section count as a side-trip.

Interesting to note is that for the sector LHR-WAW I was offered the LO code-share flight using BA, although this was only available for travel in Y class (ie C class rewards not permitted at all).

Travel between BUD and ZRH is permitted on both LX and the LX code-share with Malev.

Regards

WG