GMs: Storyline Help?

[Ordovician]

I'm running a play-by-post game on another board, and have run into a bit of a snag. The two groups of characters are in two separate threads, and I'm itching to get them together as soon as possible. The characters are two Time Lords, a psychic human, a future human who reminds me a lot of a Bernice Summerfield type, and a Kang. Basically, the situation is that the Macra (the proper intelligent Macra) are invading 1950s London for a reason which shall become apparent later (I don't really want to post this in case any of my players are here). The future human, the Kang, and an NPC are currently in the tunnels near Liverpool Street Station hunting down the Macra. The rest of the characters were investigating a factory which was a front for the Macra; the cloister bell of the TARDIS started going off, and one of the Time Lords and the psychic went after it while the other is getting interrogated by the Macra. These two are at the TARDIS now.

Basically, my problem comes in that I was first trying to get the future human and the Kang to leave the station and run into the Time Lord and the psychic as they drew up to the TARDIS, but then they all chose to go further into the tunnels; then I was hoping the other two would get sent after the station group by an NPC outside, but they sort of glossed over that NPC and are getting in the TARDIS anyway. I'm currently drawing a blank as to how to bring the two groups together before they go back to the factory and get the other Time Lord. I suppose I could sort of hijack the TARDIS and have it materialize in the tunnels before it lets them go back for the other, but I'm hoping that won't seem too contrived.

Any other ideas?

[da professor]

Might the future human and/or the Kang recognise the sound of the Tardis dematerialising as it echoes through the tunnels? Even if not, it's an unusual sound they might want to investigate.

[Curufea]

The Master and the 3rd Doctor typically nicked bits from each other's TARDISes to have an excuse to be around each other

Another similar idea is a unique Maguffin that both groups need that can only be found in one place at one time.

[Rel Fexive]

The group in the TARDIS find some terrible threat approaching; the HADS moves the TARDIS somewhere safe... down in some tunnels nearby, in earshot of the tunnel investigators.