Hi there
I used to work in the West End and know that most tickets are booked through agents now. Ticket Tree is just a small one based in Hull who look reasonably OK . Here's their address and phone number if you want to ring them to check everything. ttp://www.tickettree.com/
Also if you're in any doubt ring the Theatre Royal Drury Lane and ask them to clarify your booking. Their phone number is here and they can tell you if Ticket Tree have the seats booked that you have on your yellow paper.
http://www.londontheatre.co.uk/londontheatre/w estendvenues/drurylane.htm
I'm pretty sure all is OK and the tickets will be there on the night. I presume its Oliver you're off to see, I hope you enjoy it. I should go book myself now having rooted for Jodie all the way!