MEL-LHR - One flight??
 

on a RTW ticket, does the BA flight from MEL-LHR (via) SIN count as a single flight if you stay with the same flight number the whole way without a stop-over in SIN? i.e get the BA18. I know with this flight you would get 330 BA tier points as long as you dont take the stop-over (in which case you would get 2x 180 Tier points in F), so does it count as a single sector when calculating the 20 sector max, or is it still classed as 2 sectors?

Yes it is one segment if you take it as described.

Although it counts as a single sector it does not avoid having to pay for an extra continenet e.g. LHR-MEL-LAX-LHR is not three continents (Europe-Australia-US-Europe) but four (Europe-Asia-Australia-US-Europe).

From the Rules:-

Extra continent
 

Just to clarify the point, if you have already gone North America-Asia then Australia, you would not get fined for an extra continent as you can pass through Asia twice if you are taking a flight such as BA 18.

If you had the coupons/sectors available, then you could always switch from the BA to the QF half of the aircraft for the second leg and get the extra status points if you need them. Very useful if you are (a) short on points for one card or (b) in possession of multiple 1W FF cards.

It would be as part of a AONE4, going Europe-USA-Asia-SWP-Europe, so not a problem for the 4 continent rule - it was mainly for the sectors - i'm trying to fit in as many sectors as possible, and as I can do MEL-LHR as a single sector on one flight number (BA18), this is better than using 2 sectors to do MEL-SIN-LHR on the same flight (despite dropping a whole 30 Tier points and only earning 330, instead of 2x180) :)

It's all currently a flight of fancy, but thanks for the help.