We are going to be in London in late Sept. I have always wanted to see the play, Mousetrap, and so really want to make sure I get to do so while there. I need a bit of quick advice as to tickets - should I buy them online before leaving Australia (to make sure we can get in and maybe not pay the highest priced seats) or will we be able to get decent priced seats 'on the day'? Would love to hear some opinions.

Looks like you can tickets a day or two out or day of at the TKTS booth in Leicester Square http://www.tkts.co.uk/whats-on-sale/

I would suggest securing tickets before you leave home to avoid being disappointed. Last fall, we decided to wait until we arrived in London to get tickets for Mousetrap. We were fortunate to get tickets for a mid-week matinee performance. Seats were very limited. It's been playing in London for over 60 years & there's a reason why it's still so popular. It's classic Agatha Christie brilliantly done! You will enjoy it!

St Martins is quite a small theatre, which contributes to tickets selling out quickly at peak times. However, we went to the Mousetrap at the beginning of July on a Thursday evening and thoroughly enjoyed it, but the theatre wasn't full. We booked our tickets online some 6 weeks before the show.

Pidge51, I am so glad that you posted this. We are also going in September and are going to book online.
I saw this in 1972 (yikes) and have decided it's about time to see it again! Hope you have a wonderful time.