Thanks for info everyone.

I understand that single fares are not hard to get, but I was surprised that there wasn't a fare that bolted on to the trans-mongolian. Afterall, all the Oneworld segments are there. I am going to use points to help with some of the segments.

As for the train, I have the train part sorted and technically the trans-mongolian does leave Moscow, but I'm starting the train journey at St Pertersburg - a common choice. I am taking about 21 days and I was originally going to travel London to Tibet by train, but decided this may being a bit much over a short period of time.

For anyone interested, the trip I am taking: St Petersburg-Moscow-Suzdal-Irkutsk-Ulaan Baatar-Terelj Park-Beijing