Thanks again.
After a fairly long-winded e-mail correspondence with the CTA, I've asked them to advise on what they would have been able to book given that my earlier suggestion was clearly not good enough and, to my amazement, this is what I got -
FRI 6/7
Premium Economy, Full Flex, A1FFX0 (~550 EUR)
LO281 Warsaw/WAW-London/LHR
SAT 7/7
Economy Flex (~700 EUR)
OS454 London/LHR-Vienna/VIE
OS759 Vienna/VIE-Sarajevo/SJJ
They claimed this was the best available fare as they were unable to combine the two segments (multi-city) and so they had to book them separately (i.e. 2 one-way tickets).
The suggestion box at head office will soon get a note from myself suggesting to look at alternatives in terms of travel agent...
G