I've just provisionally booked a Qatar Business Class OSL-DOH-SIN return for approx £2310 for 2 passengers - £1155 each. That's 560 tier points / £2.06 per tier point I think.
Dates are May 3rd to May 7th.
I now need 4 BA flights and 40 tier points to get to 600 and was considering flying MAN-LHR-OSL as a positioning flight, but I'm feeling nervous about a) relying on trains to Manchester unless I stay the night before and b) having complex positioning flights, even if I plan an overnight stay in OSL.
So I think I'll just fly LHR-OSL-LHR on BA, arriving the day before the outgoing flights, and find a dead-cheap BA return flight on another occasion.