What +MTWO (range) your B747-300 has?
Edit: EGLL - YMML are 9127nm, the perfect tech stop would be at 4563nm, but even the biggest +MTOW variant of the B747-300 can't reach that... (I took the range out of the AWS compare page)
However, these are good airports for technical stops;
Mandalay International, Myanmar (24h open) - 9128nm (4632nm + 4496nm)
Kathmandu, Nepal (24h open) - 9127 (5146nm + 3981nm)